Sahih Muslim Book 3, Hadith Number 727.

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?