Musnad Ahmad

Jabir bin Subh says that Mathna bin Abdur Rahman, whose companionship I had up to Wasit, used to say "Bismillah fi awwalihi wa akhirihi" at the beginning and at the last morsel of food. Once I asked him, "You recite Bismillah at the beginning of the meal, but what is the need to say it at the last morsel?" He replied, "I will tell you the reason for this. I heard my grandfather, Hazrat Umayyah bin Makhshi, who was among the companions of the Prophet (peace be upon him), say that once a man was eating food while the Prophet (peace be upon him) was watching him. He did not recite Bismillah. When he reached the last morsel, he remembered that he had not recited Bismillah, so he said, 'Bismillah awwalihi wa akhirihi.' Upon hearing this, the Prophet said, 'Satan kept eating with him, but when he recited Bismillah, Satan vomited out whatever was in his stomach.'"