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 (peace_be_upon_him), 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 Apostle of Allah (peace_be_upon_him) say: This is the seat of the devil, referring to the back knot of the hair. (Sunan Abu Da'ud 2.646)
Narrated Abdullah ibn Abbas: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 Apostle of Allah (peace_be_upon_him) say: A man who prays with the black knot of hair tied is the one praying pinioned. (Sunan Abu Da'ud 2.647)