From Sahih Bukhari (Hadith 3558):
Narrated Ibn `Abbas:Allah's Messenger (ﷺ) used to let his hair hang down while the infidels used to part their hair. The people of the Scriptures were used to letting their hair hang down and Allah's Messenger (ﷺ) liked to follow the people of the Scriptures in the matters about which he was not instructed otherwise. Then Allah's Messenger (ﷺ) parted his hair
Reference: Bukhari 3558