From Sahih Bukhari (Hadith 5917):
Narrated Ibn `Abbas:The Prophet (ﷺ) used to copy the people of the Scriptures in matters in which there was no order from Allah. The people of the Scripture used to let their hair hang down while the pagans used to part their hair. So the Prophet (ﷺ) let his hair hang down first, but later on he parted it
Reference: Bukhari 5917