Benefits of the Hadith: (… from Ibn 'Abbas (may Allah be pleased with him and his father), who said:

The Shaykh : Regarding letting the hair hang down: When the Prophet (peace and blessings be upon him) first arrived in Madinah, he liked to agree with the People of the Book in matters in which he had not been commanded to do the opposite. They used to let their hair hang down, meaning they would direct their hair in one direction toward the back, so he would do the same as them. The polytheists, on the other hand, used to part their hair. So the Prophet (peace and blessings be upon him) was between two options: either agree with the polytheists or agree with the People of the Book? It is well known that agreeing with the People of the Book is closer to the truth, because they are upon a Book, unlike the polytheists. So he used to let his hair hang down. Then, when the people embraced Islam in the end, he (peace and blessings be upon him) began to part his hair, because the description that he liked to distance himself from, which was shirk (associating partners with Allah), had been removed. So he began to part his hair. Thus, the Sunnah for one who has a head of hair, meaning one who has grown hair, is to part it, meaning to divide his hair into two sections, one section to the right and one section to the left.