Abu Dawud Book 2, Hadith Number 646.
Chapter : Not known.
Narated By 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 (PBUH), 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 (PBUH) say: This is the seat of the devil, referring to the back knot of the hair.