حَدَّثَنَا يَعْقُوبُ حَدَّثَنِي أَبِي عَنِ الزُّهْرِيِّ عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ عَنِ ابْنِ عَبَّاسٍ أَنَّهُ قَالَ كَانَ أَهْلُ الْكِتَابِ يَسْدِلُونَ أَشْعَارَهُمْ وَكَانَ الْمُشْرِكُونَ يَفْرُقُونَ رُءُوسَهُمْ قَالَ وَكَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُعْجِبُهُ مُوَافَقَةُ أَهْلِ الْكِتَابِ فِي بَعْضِ مَا لَمْ يُؤْمَرْ بِهِ فِيهِ فَسَدَلَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ نَاصِيَتَهُ ثُمَّ فَرَقَ بَعْدُ
مسند امام احمد بن حنبل · Hadith 2364
It was narrated from Ibn 'Abbas (RA) that he said: The people of the Book used to let their hair down and the mushrikeen used to part their hair. The Messenger of Allah liked to do some of what the people of the Book did in some matters concerning which he had not received any commands, so the Messenger of Allah (PBUH) let his forelock down. Then later on he parted his hair.
Sanad
حَدَّثَنَا يَعْقُوبُ حَدَّثَنِي أَبِي عَنِ الزُّهْرِيِّ عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ عَنِ ابْنِ عَبَّاسٍ