Sahih Muslim Book 003, Hadith Number 0727.
Chapter : It is permissible to eat without ablution and there is no abhorrence in it and performing of ablution immediately (after that) is not essential.
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?