Sunan Abi Dawud

Narrated 'Abd Allaah b. Muhammad:We were in the company of 'Aishah. When her food was brought in, al-Qasim stood up to say his prayer. Thereupon , 'Aishah said : I heard the Messenger of Allaah (sal Allaahu alayhi wa sallam) say: Prayer should not be offered in presence of meals, nor at the moment when one is struggling with two evils (e.g. when one is feeling the call of nature)