Musnad Ahmad 1153:436

Sulayman ibn Dawud narrated to us, Isma'il narrated to us, he said: Abu Hazrah al-Qass narrated to me, from 'Abdullah ibn Abi 'Atiq, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, said: "None of you should pray while food is present, nor while he is being suppressed by the two impurities (urine and feces)."