Kurayb, the mawla (freedman) of Sayyiduna Ibn Abbas (may Allah be pleased with them both), says that Sayyiduna Ibn Abbas (may Allah be pleased with them both) saw Abdullah bin al-Harith praying with the hair of his head gathered and tied at the back. He (may Allah be pleased with him) stood behind him and began to untie his hair. When he finished the prayer, he turned to Sayyiduna Ibn Abbas (may Allah be pleased with them both) and said: What business do you have with my head? He said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "The likeness of one who prays with his hair tied in a knot is like one who prays 'maktuf' (with his arms tied behind him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2679