Sunan Abu Dawood, Chapter: Prayer (Kitab Al-Salat)

Said ibn Abu Said al-Maqburi reported on the authority of his father that he saw Abu Rafi 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. Abu Rafi untied it. Hasan turned to him with anger, Abu Rafi 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.