Praise be to Allah.

Al-Bukhaari (3551) and Muslim (2337) narrated that al-Bara’ said: The Messenger of Allah (blessings and peace of Allah be upon him) was a man of medium height, broad shouldered with thick hair hanging down to his earlobes.

According to a report narrated by Muslim: I have never seen any man with long hair more handsome in a red suit than the Messenger of Allah (blessings and peace of Allah be upon him), with his hair reaching his shoulders. He was broad shouldered …

Al-Bukhaari (5903) and Muslim (2338) narrated from Anas that the hair of the Messenger of Allah (blessings and peace of Allah be upon him) came down to his shoulders.

Abu Dawood (4187) narrated that ‘Aa’ishah said: The hair of the Messenger of Allah (blessings and peace of Allah be upon him) came between his earlobes and his shoulders.

Classed as saheeh by al-Albaani in Saheeh Abi Dawood.

Al-Mannaawi (may Allah have mercy on him) said:

Abu Shaamah said: The saheeh reports indicate that his hair came to halfway down his ears; according to one report it reached his earlobes, and according to other reports it came between his earlobes and his shoulders, or it came down to his shoulders. No report has reached us to suggest that it was ever longer than that, and these differences reflect differences in circumstances. These reports reflect different circumstances, and he shaved his head in the case of Hajj or ‘umrah. End quote.

Fayd al-Qadeer (5/74)

Abu Dawood (4191) narrated that Umm Haani’ (may Allah be pleased with her) said: The Prophet (blessings and peace of Allah be upon him) came to Makkah with four braids.

Classed as saheeh by al-Albaani in Saheeh Abi Dawood.

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:

What the hadith indicates is that the hair of the Prophet (blessings and peace of Allah be upon him) was almost shoulder length in most cases, and that sometimes it might become so long that he would braid it. This is to be understood as referring to situations in which a long time passed during which he paid little attention to his hair, such as when he was very busy or travelling and the like.

End quote from Fath al-Baari (10/360)

The hair of the Prophet (blessings and peace of Allah be upon him) came to above his shoulders and below his earlobes. His hair reached his earlobes, and when it grew long he would wear it in four braids. Umm Haani’ (may Allah be pleased with her) said: The Messenger of Allah (blessings and peace of Allah be upon him) came to us in Makkah with four braids. This is a saheeh hadith. End quote.

Zaad al-Ma‘aad (1/170)

Conclusion:

When his hair grew long because of travel or other reasons, the Prophet (blessings and peace of Allah be upon him) would put it into four braids, which was in accordance with the custom of the Arabs. So he would put his hair back, then make it into strands and twist them together, then they would become four braids.

However we should note that letting the hair grow long is not in and of itself part of the Sunnah for which a Muslim will be rewarded, because this comes under the heading of traditions and customs. The Prophet (blessings and peace of Allah be upon him) both let his hair grow long and shaved it, and there is no reward in letting it grow long and no sin in shaving it. Rather he enjoined us to take care of the hair, and he did not do anything (with regard to his hair) that was contrary to the custom of the Arabs.

If it so happens that customs change in some time or place, and having long hair becomes something that is only for women, then it is not permissible for men to let their hair grow long in that place. And if that becomes a sign of evildoers or fools, then people of good character and dignity should not do what they do or adopt their ways.