Ibn'Abbaas Radiyallahu'Anhu says:
"Once when Rasulullah Sallailahu 'Alayhi Wasallam came out of the toilet, after relieving himself from the call of nature, food was served to him. They asked if water for wudu should be brought? He replied: 'I have been commanded to perform wudu when 1 perform salaah'".
English reference: Book 26, Hadith 176
Arabic reference: Book 27, Hadith 185