The infallibility of the Prophets, may Allah exalt their mention, is an established fact based on reason and tradition. Reason requires the infallibility of the Prophets, may Allah exalt their mention, because the Prophets came to convey to people the Message of Allah. If we liken this Message or the Divine Revelation to light or pure water, as the Quran itself does [chapter 13. verse17 and chapter 24. verse 3], it is absolutely necessary and indispensable to the nature of the Revelation that both the Jibreel (Archangel Gabriel), who brought the Revelation, and the Prophet 
himself, who conveyed it to people, should be absolutely pure. Otherwise, that Divine light, the Revelation, would have been extinguished or dimmed, or that ‘pure water’ polluted.
Every falling off is an impurity, a dark spot, in the heart. Like Jibreel, the heart or soul of the Prophet is like a ‘polished mirror’ through which the Divine Revelation is reflected to people, or a ‘cup’ from which people quench their thirst for that pure ‘Divine water’. Any black spot on the mirror would absorb a ray of that light; a single drop of mud would be enough to make the water unclear. This would mean that the Prophets, may Allah exalt their mention, did not - Allah forbid such a thought! - convey the whole of Allah’s Divine Message. Whereas, in truth, they, may Allah exalt their mention, performed their duty perfectly and left nothing of the Message unconveyed. This is clear from the following Quranic verses (which mean): "O Messenger, announce that which has been revealed to you from your Lord, and if you do not, then you have not conveyed His message. And Allah will protect you from the people. Indeed, Allah does not guide the disbelieving people." [Quran 5:67]
Another verse Says (what means): "This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion." [Quran 5:3]
People learn from the Prophets all the commandments and principles concerning belief and conduct. In order that people should learn these commandments in their pristine purity and truth and as perfectly as possible to secure their happiness and prosperity in both worlds, the Prophets, may Allah exalt their mention, must, first, represent, and, then, present them without any faults or defects, for they, may Allah exalt their mention, are guides and good examples for people to follow, as explicitly stated in the verses (which mean): "There has certainly been for you in the Messenger of Allah an excellent pattern for anyone whose hope is in Allah and the Last Day and [who] remembers Allah often." [Quran 33:21] and also in the following verse (which means): "There has already been for you an excellent pattern in Abraham and those with him […] There has certainly been for you in them an excellent pattern for anyone whose hope is in Allah and the Last Day." [Quran 60: 4, 6]
Despite his utmost care not to do anything contrary to Islam and not even to say a single word which is not sanctioned by Allah, if a Prophet were to utter an untrue word, he would repent for a lifetime, or even longer. It is narrated that the Prophet Ibraheem (Abraham) 
will direct to Moosa (Moses) those who will appeal to him to intercede for them on the Day of Judgment, saying he cannot as he spoke allusively three times in his life. [Muslim]
Although it is not a sin to make an ‘indirect’ reference to the truth when it is more appropriate rather than being direct, Prophet Ibraheem’s repentance of his three allusions will continue in the Hereafter.
The Quran commands believers to obey all the orders or prohibitions of the Prophet without exception, and emphasizes that it is not fitting for a believer, man or woman, when a matter has been decided by Allah and His Messenger to have any option about their decision. [Quran 33:36]
It also warns believers that when Allah and His Messenger have given a judgment they should only say: "We hear and we obey." [Quran 24:51]. Absolute obedience to a Prophet means that the Prophet is right in all his commands and prohibitions.
Prophethood is so great a favor that all the Prophets, may Allah exalt their mention, bore unbearable pains in fulfilling the duty of thanksgiving and were always worried about not having worshipped Allah sufficiently. Prophet Muhammad often implored Almighty Allah using the following words: "Glory be to You, we have not been able to know You as Your knowledge requires, O Known One.
Glory be to You, we have not been able to worship You as Your worship requires, O Worshipped One."
The Quranic verses which are sometimes mistakenly understood to reprimand certain Prophets for some faults of theirs, or to mean the Prophets sought Allah’s forgiveness for some sin of theirs, should be considered from this point of view. Besides, Allah’s forgiveness does not always mean that a sin has been committed. The Quranic words of ‘afw - ‘pardon’, and maghfirah - ‘forgiveness’ also mean ‘special favor and kindness and Divine dispensation in respect to the lightening or the overlooking of a religious duty’, as in the following verses (which mean):
"But whoever is forced by severe hunger with no inclination to sin — then indeed, Allah is Forgiving and Merciful." [Quran 5:3]
"If... you find no water, then seek clean earth and wipe over your faces and your hands [with it]. Indeed, Allah is ever Pardoning and Forgiving." [Quran 4:43]
Sins and pardon have different degrees:
· Sins committed by not obeying the religious commandments, and the forgiveness thereof;
· Sins committed by disobeying Allah’s laws of creation and life, and the forgiveness thereof;
· Sins in respect of behaving against the rules of good manners or courtesy, and the forgiveness thereof.
A fourth type which is not a sin, is doing something good but not the best, a failure in doing perfectly what is required by the love of, and nearness to, Allah. This is what some of the Prophets may have done, so it is not a sin in our normal usage of the word for something deserving of Divine punishment.
Tradition also proves the Prophets’ infallibility.
Almighty Allah Says in the Quran concerning Prophet Moosa (what means):
"And I bestowed upon you love from Me that you would be brought up under My eye [i.e. observation and care]." [Quran 20:39]
Prophet Moosa was brought up by Allah Himself and prepared for the mission of Messengership. Therefore, it is inconceivable that he may have committed a sin at any time in his life.
The same is true of all the other Prophets, may Allah exalt their mention. For example, Prophet Muhammad said of Jesus:
"Satan could not touch 'Eesa (Jesus) and his mother at his birth. Jesus was protected from birth until his elevation to the Presence of Allah, as we also read in the Quran (what means):
"So she [(Mariam (Mary)] pointed to him. They said, “How can we speak to one who is in the cradle a child?” ['Eesa] said, “Indeed, I am the servant of Allah. He has given me the Scripture and made me a prophet. And He has made me blessed wherever I am and has enjoined upon me prayer and Zakah (poor-due) as long as I remain alive. And [made me] dutiful to my mother, and He has not made me a wretched tyrant. And peace is on me the day I was born and the day I will die and the day I am raised alive.” [Quran 19: 29-33]
'Eesa, like all the other Prophets, was protected from all kinds of sin from his birth. Prophet Muhammad intended in his childhood to attend two wedding ceremonies at different times but, on each occasion, he was overpowered by sleep which prevented him from attending. [Ibn Katheer, Al-Bidayah, vol. 2, p.350-1]
Likewise, in his youth, he helped his uncles with the restoration of the Ka’bah (Sacred House in Makkah) by carrying stones. Since the stones hurt his shoulders, his uncle, Al-‘Abbas 
advised him to hoist the garment covering the lower part of his body, onto his shoulder to carry the stones on. He just did what he was advised to do, leaving some of the upper part of his legs uncovered, when he fell on his back with his eyes staring fixedly. An angel appeared and warned him that what he had done was improper, saying: 'This is not befitting for you.' [Al-Bukhari]
For the day was to come when he would order people to be well-mannered and observe divinely ordained standards of conduct, including covering the thighs.
Allah’s Messenger says that all the children of Adam make faults and err, and the best of those who make faults and err are the repentant. [At-Tirmithi] This implies that man is fallible by nature, but it does not mean that all of mankind are ‘condemned’ to erring howsoever. Whether by Allah’s Will and special protection or, as will be explained below, by His showing the way to be free from errors or sins, even the greatest of Imams (religious leaders or scholars) who continue the Prophetic mission of guiding people may be infallible to some degree.
Allah promises to protect the believers who fear Him, and to endow them with sound judgment to enable them to distinguish between truth and falsehood, and between right and wrong. Allah, the Most Exalted, Says (what means):
"And know that your properties and your children are but a trial and that Allah has with Him a great reward." [Quran 8.29]
Allah made a covenant with the believers that if they obey Him, assist His cause and strive to exalt His Word, by proclaiming His religion, He will help them and make their feet firm in the religion, protecting them against all kinds of deviation." [Quran 47.7] Allah’s protection of believers from their enemies and against committing sins has been made dependent on their support of Islam and struggle to spread it all over the world so that only Allah is worshipped and no partners are associated with Him either in belief or worship or the creation and rule of the universe. If believers fulfill their covenant with Allah, Allah will fulfill His covenant with them [Quran 2.40]. If, by contrast, they break their promise, Allah will not make them successful [Quran17.8]
Allah protects His servants against sins in different ways. He may put some obstacles in their way to sins so they do not sin, or He may establish a ‘warner’ in their hearts, or, if all the other means prove of no use, He may cause, for example, their legs to be broken or their hands unable to hold or grasp. Or He may warn one by putting a verse in his mouth, as He did with a young man during the Caliphate of ‘Umar .
The young man was so strict and attentive in his worship that he performed all his prayers in the mosque. A woman lived on his way to the mosque and tried her hardest for several days to seduce him into making love with her. Although the young man resisted her alluring gestures, the moment came when he took a few steps towards the woman’s house. Just at this point, he felt he was reciting this verse (which means):
"Indeed, those who fear Allah — when an impulse touches them from Satan they remember [Him-Allah] and at once they have insight." [Quran 7.201]
In the face of this Divine warning, the young man was so ashamed before Allah of what he was about to do, and felt so overwhelmed by his Compassionate Lord’s preventing him from committing a sin, that he died. When ‘Umar was informed of the incident a few days later, he went to his grave and shouted: ‘O young man! "For him who fears the time when he will stand before his Lord, there will be two gardens!" [Quran 55.46] A voice from the grave, whether belonging to the young man himself or an angel on his behalf, replied: ‘O Commander of the Believers: Allah has granted me the double of what you say!' [Tafseer Ibn Katheer, vol. 3, p. 539]
This is Allah’s protection of His sincere servants. He says in one of His Revelations outside the Quran (Hadeeth Qudsi):
"My servant cannot draw near to me through something else more lovable to Me than the obligations I have enjoined upon him. Apart from those obligations, he continues to draw near to Me through supererogatory acts of worship, until I love him. When I love him, I will be his ears with which he hears, his eyes with which he sees, his hands with which he grasps, and his feet on which he walks. If he asks Me something, I will immediately give it to him; if he seeks refuge in Me from something, I will protect him from it." [Al-Bukhari]
Allah guides His true servant to good and protects him from all kinds of evil. The servant wills and does what is good and refrains from wickedness. He asks Allah what is good and whatever he asks is provided for him; he seeks refuge in Allah from what is bad, and whatever he seeks refuge in Allah from, he is protected against it.
All the Prophets were infallible. They never committed a sin, minor or major, and their lives were spent doing virtuous deeds.
