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.  

أَنَّهُ رَأَى أَبَا رَافِعٍ مَوْلَى النَّبِيِّ ﷺ 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 ﷺ يَقُولُ ذَلِكَ كِفْلُ الشَّيْطَانِ يَعْنِي مَقْعَدَ الشَّيْطَانِ يَعْنِي مَغْرِزَ ضُفُرِهِ  

Kurayb the freed slave of Ibn Abbas reported: Abdullah ibn Abbas saw Abdullah ibn al-Harith praying having the back knot of the hair. He stood behind him and began to untie it. He remained standing unmoved (stationary). When he finished his prayer he came to Ibn Abbas and said to him: What were you doing with my head? He said: I heard the Messenger of Allah ﷺ say: A man who prays with the black knot of hair tied is the one praying pinioned.  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ إِنَّمَا مَثَلُ هَذَا مَثَلُ الَّذِي يُصَلِّي وَهُوَ مَكْتُوفٌ