From Sahih Bukhari (Hadith 3944):
Narrated `Abdullah bin `Abbas:The Prophet (ﷺ) used to keep his hair falling loose while the pagans used to part their hair, and the People of the Scriptures used to keep their hair falling loose, and the Prophet (ﷺ) liked to follow the People of the Scriptures in matters about which he had not been instructed differently, but later on the Prophet (ﷺ) started parting his hair
Reference: Bukhari 3944