Hadith
حَدَّثَنَا الْحَسَنُ بْنُ أَبِي عَلِيٍّ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، عَنِ ابْنِ جُرَيْجٍ، حَدَّثَنِي عِمْرَانُ بْنُ مُوسَى، عَنْ سَعِيدِ بْنِ أَبِي سَعِيدٍ الْمَقْبُرِيِّ، يُحَدِّثُ عَنْ أَبِيهِ، أَنَّهُ رَأَى أَبَا رَافِعٍ مَوْلَى النَّبِيِّ صلى الله عليه وسلم مَرَّ بِحَسَنِ بْنِ عَلِيٍّ عَلَيْهِمَا السَّلاَمُ وَهُوَ يُصَلِّي قَائِمًا وَقَدْ غَرَزَ ضُفُرَهُ فِي قَفَاهُ فَحَلَّهَا أَبُو رَافِعٍ فَالْتَفَتَ حَسَنٌ إِلَيْهِ مُغْضَبًا فَقَالَ أَبُو رَافِعٍ أَقْبِلْ عَلَى صَلاَتِكَ وَلاَ تَغْضَبْ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " ذَلِكَ كِفْلُ الشَّيْطَانِ " . يَعْنِي مَقْعَدَ الشَّيْطَانِ يَعْنِي مَغْرِزَ ضُفُرِهِ .
Narrated AbuRafi': Sa'id ibn AbuSa'id al-Maqburi reported on the authority of his father that he saw AbuRafi' the freed slave of the Prophet (ﷺ), passing by Hasan ibn Ali (Allah be pleased with them) when he was standing offering his prayer. He had tied the back knot of his hair. AbuRafi' untied it. Hasan turned to him with anger, AbuRafi' said to him: Concentrate on your prayer and do not be angry: I heard the Messenger of Allah (ﷺ) say: This is the seat of the devil, referring to the back knot of the hair
Hasan · Al-Albani
Sunan Abi Dawud 646 ·Book 2, Hadith 256·Also available on sunnah.com·