Abu Dawud Book 28, Hadith Number 4176.

Chapter : Parting of the hair.

Narated By Ibn Abbas : The people of the Book used to let their hair hang down, and the polytheists used to part their hair. The Apostle of Allah (PBUH) liked to conform with the People of the Book in matters about which he had received no command. Hence the Apostle of Allah (PBUH) let his forelock hang down but afterwards he parted it.