It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both): We were with the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) went to the toilet, then he came back, and food was brought to him. It was said: "O Messenger of Allah, will you not perform ablution?" He (peace and blessings be upon him) said: "For what? When I pray, then I will perform ablution."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 189