سنن أبي داود

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 Messenger of Allah (ﷺ) say: A man who prays with the black knot of hair tied is the one praying pinioned.