Mukhtasar Muslim

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them) that he saw Sayyiduna Abdullah bin Harith praying with his hair tied in a knot, so Sayyiduna Abdullah bin Abbas (may Allah be pleased with them) began to untie his knot. When he finished the prayer, he asked, "Why did you touch my head?" Sayyiduna Abdullah bin Abbas (may Allah be pleased with them) said, "I heard the Messenger of Allah (peace and blessings be upon him) say: Whoever prays with his hair tied in a knot, it is as if he prays with his private parts uncovered."