Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 89
Narated By 'Abd Allah b. Muhammad : We were in the company of 'Aisha. When her food was brought in, al-Qasim stoodd to say his prayer. Thereupon, 'Aisha said: I heard the Messenger (pbuh) say: Prayer should not be offered in the presence of meals, nor at the moment when one is struggling with the two evils (i.e. when one is feeling the call of nature).