`Abdullah ibn `Abbas reported that he saw `Abdullah ibn Al Harith offering Salah while (his hair) was plaited behind his head. So, he (`Abdullah ibn `Abbas) stood up and unfolded them. When he finished Salah, he approached Ibn `Abbas and said to him: Why is it that you touched my head? He (Ibn `Abbas) replied: I heard the Messenger of Allah (peace be upon him) saying: (The man who offers Salah with plaited hair) is like the one who offers Salah with his hands tied behind. 
The meaning of the Hadith:
Imam An-Nawawy said: Scholars have agreed that Salah is forbidden while a person lifting his garment or his sleeves, or his hair is plaited, or covered under his turban and so on. All these actions are forbidden according to the consensus of scholars, however if a person offers Salah in these conditions, he has done wrong but his Salah is valid. 
Lessons gained from the Hadith:
1- The eagerness of the Companions (may Allah be pleased with them) on conveying knowledge which they knew or memorized from the Messenger of Allah (peace be upon him).
2- The permissibility of changing evil things with one's hand, particularly when a person is able to do so and as long as no bigger harm is done, otherwise changing evil actions should be done by tongue then denying the action by one's heart.