Sahih Muslim Book 4, Hadith Number 996.

Chapter : The excellence of prostration and exhortation to observe it.

Abdullah b. Abbas reported that he saw ‘Abdullah b. al-Harith observing the prayer and (his hair) was plaited behind his head. He (‘Abdullah b. ‘Abbas) stood up and unfolded them. While going back (from the prayer) he met Ibn ‘Abbas and said to him: Why is it that you touched my head? He (Ibn ‘Abbas) replied: (The man who observes prayer with plaited hair) is like one who prays with his hands tied behind.