Musnad Ahmad 1153:2071

Rawh narrated to us, he said: Hisham bin Abi Abdullah narrated to us, from Budayl, from Abdullah bin Ubayd bin Umayr al-Laythi, from a woman among them who is called Umm Kulthum, from Aisha, that the Messenger of Allah (peace and blessings of Allah be upon him) was eating with six of his companions. A hungry Bedouin came and ate [the food] in two morsels. The Prophet (peace and blessings of Allah be upon him) said: "As for him, had he mentioned the name of Allah, it would have sufficed you all. So when one of you eats, let him mention the name of Allah. If he forgets to mention the name of Allah at its beginning, let him say: 'In the name of Allah at its beginning and its end.'"