Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 727
Ibn 'Abbas reported: The Messenger of Allah (may peace be upon him) went to the privy and when he came back, he was presented with food. It was said to him; Messenger of Allah, wouldn't you perform ablution. He said: Why, am I to say prayer?