Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Prophet (peace and blessings be upon him) came out from relieving himself, then sat down and ate. The people said: O Messenger of Allah! Will you not perform ablution? He (peace and blessings be upon him) said: "I have only been commanded to perform ablution for prayer; as for food, then no."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12494