Sayyiduna Ibn Abbas (may Allah be pleased with them both) saw Sayyiduna Abdullah bin al-Harith (may Allah be pleased with him) praying while his hair was tied up behind him. He stood behind him and began to undo (the hair), and the other let him do so, until, having finished the prayer, he said: "What business did you have with my head?" Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "I heard the Messenger of Allah (peace and blessings be upon him) say: 'The likeness of one who prays with his hair tied up is that of one who prays with his hands tied.'"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12145