#كل_الرسل_مسلمين

God sent Prophets to teach mankind how to worship Him; starting with Adam, including Noah, Abraham, Moses, Jesus and the last of the messengers, Muhammad (peace be upon them all). The Islamic position is that all of these prophets came with the same message, that there is no deity worthy of worship except the One True God, known in Arabic as Allah. 
In Islam we believe that all the prophets were Muslims, so prophets such as Noah, Abraham, Moses, and Jesus (peace be upon them all) were all Muslims. Some people might ask how were they Muslims, when Islam began 1400 years ago with the Prophet Muhammad (pbuh) in the desert of Arabia? 
 
Some clarification is needed here, as Muslims we believe that the Quran was revealed to the Prophet Muhammad (pbuh), and that he was the last and final messenger, we do not believe that Islam started with him. Islam started from the first prophet, Adam, and continued until the Prophet Muhammad. 
 
All of these prophets we believe, represented the same God, the one and true God that had sent them, to preach to the masses, and to allow the masses to know their creator. That’s why as Muslims we say we believe in the same God as Jews and Christians; we believe that the source for Moses and Jesus, was the same source, the same God for Muhammad peace be upon them all. 
 
All of the prophets were a continuation of each other, each sent to their own people and tribe, with the same message of teaching and correcting the people’s understanding of the one and true God. 
 
So Noah was sent to his own people, Moses was sent to his own people, Jesus was sent to his own people, and all of them were sent by the same God, the same God that sent the Prophet Muhammad (pbuh) with the same message.

#الاختلاف_بين_الرسل

The only differences between the prophets was in the revelation they were given, not the contents of the main message. For example Moses was given the Torah, Jesus the Gospel, David the Psalms, and lastly the Prophet Muhammad was given the Quran. So they were given different books by God, but with the same underlying message about the one true God. 
 
Other differences between the prophets was also on the rules and regulations, so for example some of the rules and regulations, i.e. the law, given to Moses for the Israelites, differed with the rules and regulations given to Jesus, and then lastly the rules and regulations given to the Prophet Muhammad was different to that of Moses and Jesus. But even though some of the rules and regulations were different between the prophets, overall the regulations and rules are quite similar; one only has to look at kosher laws with that of the Sharia, to find the many similarities in the rules and regulations between Judaism and Islam. 
 
So in essence we do not believe the prophets brought different religions or different messages, all of them were given the same basic message, but in different forms (Torah, Gospel, Quran), and at different times, and to different groups of people. 
 
We as Muslims believe that all prophets are linked together in a united brotherhood, not separate or in competition with each other, but all complimenting each other. As the Prophet Muhammad (pbuh) described the prophets, they are the bricks that complete the house, all together and united. 
 
So this is what Muslims mean when we say the prophets of old were Muslims, yes, some of their customs and laws would have been different, they may have not prayed 5 times a day, but they did believe in the same message, and in the same God.