Musnad Ahmad 25:699

'Abd al-Razzaq narrated to us, he said: Sufyan informed us, from 'Amr ibn Dinar, from Sa'id ibn al-Huwayrith, from Ibn 'Abbas, he said: The Prophet, peace and blessings of Allah be upon him, went to relieve himself, and he fulfilled his need. Then food was brought near to him, and they said: "Shall we bring you water for ablution?" He said: "For what thing should I perform ablution? Am I to pray, so that I perform ablution?" — or "Have I prayed, so that I perform ablution?"