Musnad Abu Ya'la al-Mawsili 150:300

Musa narrated to us, Yahya narrated to us, Abu Hazrah narrated to us, Abdullah bin Muhammad narrated to us, saying: We were with Aisha when food was brought, and al-Qasim stood up to pray. She said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "There is no prayer when food is present, nor while one is suppressing the two impurities (urine and feces)."