Musnad Ahmad 1153:1715

Waki’ narrated to us, he said: Hisham Sahib al-Dastawa’i narrated to us, from Badil ibn Maysarah, from Abdullah ibn Ubayd ibn Umayr, from a woman who is called Umm Kulthum, from Aisha, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "If one of you eats food, then let him say: 'In the name of Allah' (Bismillah). If he forgets at the beginning of it, then let him say: 'In the name of Allah at its beginning and its end' (Bismillah fi awwalihi wa akhirihi)."