Musnad Ahmad 25:690

'Affan narrated to us, Wuhayb narrated to us, Ayyub narrated to us, from Ibn Abi Mulaykah, from Ibn 'Abbas: That the Prophet, peace and blessings of Allah be upon him, came out from the privy, and food was brought to him. It was said to him: "Will you not perform ablution?" He said: "I have only been commanded to perform ablution when I stand up for prayer."