Musnad Ahmad 25:1043

Hajjaj narrated to us, Layth informed us, ‘Amr ibn al-Harith narrated to us, from Bukayr ibn ‘Abd Allah, from Shu‘bah, the freed slave of Ibn ‘Abbas—or Kurayb, the freed slave of Ibn ‘Abbas—that ‘Abd Allah ibn ‘Abbas passed by ‘Abd Allah ibn al-Harith ibn Abi Rabi‘ah while he was praying with his hair braided and tied behind his head. He stood over him and did not cease until he untied the knots of his head. ‘Abd Allah ibn al-Harith allowed him to do so until he finished untying them, then he sat down. When Ibn al-Harith finished the prayer, he came to him and said: "Why did you do to my head what you just did?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The likeness of one who prays while his hair is tied behind his head is like the likeness of one who prays while he is bound.'"