Musnad Ahmad 25:1523

Isma'il narrated to us, Ayyub narrated to us, from 'Amr ibn Dinar, from Sa'id ibn al-Huwayrith, from Ibn 'Abbas: That the Messenger of Allah (peace and blessings of Allah be upon him) came out from the privy, and food was brought near to him. They said: "Shall we not bring you water for ablution?" He said: "Am I to pray, that I should perform ablution?"