Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 224
Narated By 'Aisha : When the Prophet (may peace be upon him) wanted to eat or sleep, he would perform ablution. She meant that (the Prophet did so) when he was sexually defiled.
Abu Dawud said: In the chain of this tradition there is a narrator between Yahya b. Ya'mur and 'Ammar b. Yasir. 'Ali b. Abi Talib, Ibn 'Umar and 'Abd Allah b. 'Amr said: When a person who is sexually defiled wants to eat, he should perform ablution.