Umayya b. Makhshi told that a man who was eating and had not mentioned God’s name said when he raised the last morsel to his mouth, “In the name of God at the beginning and at the end of it.” The Prophet laughed and said, “The devil kept eating along with him, but when he mentioned God’s name he vomited what was in his belly.”
‘A’isha reported God’s messenger as saying, “When one of you eats and forgets to mention God over his food he should say, ‘In the name of God at the beginning and at the end of it’.”
We were with the Prophet when food was presented to him, and I never saw food which had greater blessing when we began to eat, or less when we finished. We asked God’s messenger how that was, and he replied, “We mentioned God’s name when we ate, then one who ate sat down without mentioning God's name and the devil ate with him.”