The people of the Book used to let their hair hand down, and the polytheists used to part their hair. The Messenger of Allah ﷺ like to confirm with the People of the Book in the matters about which he had received no command. Hence he Messenger of Allah ﷺ let his forelock hang down but afterwards he parted it. (Using translation from Abū Dāʾūd 4188)
أَنَّهُ قَالَ كَانَ أَهْلُ الْكِتَابِ يَسْدِلُونَ أَشْعَارَهُمْ وَكَانَ الْمُشْرِكُونَ يَفْرُقُونَ رُءُوسَهُمْ قَالَ وَكَانَ رَسُولُ اللهِ ﷺ يُعْجِبُهُ مُوَافَقَةُ أَهْلِ الْكِتَابِ فِي بَعْضِ مَا لَمْ يُؤْمَرْ فِيهِ فَسَدَلَ رَسُولُ اللهِ ﷺ نَاصِيَتَهُثُمَّ فَرَقَ بَعْدُ