Ibn Abbas reported: The Messenger of Allah, peace and blessings be upon him, would let his hair hang down, while the idolaters would part their hair. The people of the Book would let their hair hang down and the Prophet liked to agree with the people of the Book in matters for which he received no divine command. Later, the Prophet parted his hair.
Source: Ṣaḥīḥ al-Bukhārī 3365, Ṣaḥīḥ Muslim 2236
Grade: Muttafaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Muslim
عَنْ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يَسْدِلُ شَعَرَهُ وَكَانَ الْمُشْرِكُونَ يَفْرُقُونَ رُءُوسَهُمْ فَكَانَ أَهْلُ الْكِتَابِ يَسْدِلُونَ رُءُوسَهُمْ وَكَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُحِبُّ مُوَافَقَةَ أَهْلِ الْكِتَابِ فِيمَا لَمْ يُؤْمَرْ فِيهِ بِشَيْءٍ ثُمَّ فَرَقَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ رَأْسَهُ
3365 صحيح البخاري كتاب المناقب باب صفة النبي صلى الله عليه وسلم
2236 صحيح مسلم كتاب الفضائل باب في سدل النبي صلى الله عليه وسلم شعره وفرقه