Abu Dawud Book 33, Hadith Number 4388.
Chapter : Not known.
Narated By Ali ibn AbuTalib : AbuZubyan said: A woman who had committed adultery was brought to Umar. He gave orders that she should be stoned.
Ali passed by just then. He seized her and let her go. Umar was informed of it. He said: Ask Ali to come to me. Ali came to him and said: Commander of the Faithful, you know that the Apostle of Allah (PBUH) said: There are three (people) whose actions are not recorded: A boy till he reaches puberty, a sleeper till he awakes, a lunatic till he is restored to reason. This is an idiot (mad) woman belonging to the family of so and so. Someone might have done this action with her when she suffered the fit of lunacy.
Umar said: I do not know. Ali said: I do not know.