Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 723
Hudhaifa reported: The Messenger of Allah (may peace be upon him) happened to meet him and he was (sexually) defiled, and he slipped away and took a bath and then came and said: I was (sexually) defiled. Upon this he (the Holy Prophet) remarked: A Muslim is never defiled.