Ibn 'Abbas said: “Allah’s Messenger(Allah bless him and give him peace) used to let his hair hang down , whereas the polytheists used to part the hair of their heads. The People of the Scripture also used to let the hair of their heads hang down, and he used favor emulating the People of the Scripture with respect to things about which nothing was commanded, but Allah’s Messenger (Allah bless him and give him peace) then parted the hair of his head.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا سُوَيْدُ بْنُ نَصْرٍ ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ الْمُبَارَكِ ، عَنْ يُونُسَ بْنِ يَزِيدَ ، عَنِ الزُّهْرِيِّ ، قَالَ : حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ ، عَنِ ابْنِ عَبَّاسٍ : " أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يُسْدِلُ شَعْرَهُ ، وَكَانَ الْمُشْرِكُونَ يَفْرِقُونَ رُءُوسَهُمْ ، وَكَانَ أَهْلُ الْكِتَابِ يُسْدِلُونَ رُءُوسَهُمْ ، وَكَانَ يُحِبُّ مُوَافَقَةَ أَهْلِ الْكِتَابِ فِيمَا لَمْ يُؤْمَرْ فِيهِ بِشَيْءٍ ، ثُمَّ فَرَقَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ رَأْسَهُ ".
sunnah.com