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)