Sahih Muslim Book 37, Hadith Number 6676.
Chapter : The exoneration of the slave-girl of Allah’s Apostle (may peace be upon him) from a false charge.
Anas reported that a person was charged with fornication with the slave-girl of Allah’s Messenger (may peace be upon him). Thereupon Allah’s Messenger (may peace be upon him) said to ‘Ali: Go and strike his neck. ‘Ali came to him and he found him in a well making his body cool. ‘Ali said to him: Come out, and as he took hold of his hand and brought him out, he found that his sexual organ had been cut. Hadrat ‘Ali refrained from striking his neck. He came to Allah’s Apostle (may peace be upon him) and said: Allah’s Messenger, he has not even the sexual organ with him.