According to the Holy Prophet (s.a.w.a.), when food is laid on the floor-cloth or the food tray is brought, thousands of angels descend from heaven and surround the food. When those who start their food say ‘Bismillah’, the angels pray to God to send His blessings on the eaters and to increase their daily bread and order Satan to leave them alone as he is the one who broke the law of God and therefore he cannot join them.
And when after having eaten, the eaters say ‘Alhamdolillah’, then the angels say that these people are among the thanks-givers as they have thanked God for the good food He has given them. If they do not say Bismillah while eating, then they invite Satan to have food with them and if they do not say ‘Alhamdolillah’ when they have eaten their food, then the angels remark that these people are those who are not satisfied with what good food God has given them for they have forgotten to thank God for the blessing; as such they are the cursed.
According to a reliable tradition a person complained to Imam Ja’far-e-Sadiq (a.s.) that he is always harmed when he takes a certain type of food. The Imam (a.s.) said that perhaps the man did not say Bismillah. But the man said that he did. At that the Imam (a.s.) replied that perhaps he talked while taking food and did not recite Bismillah when he restarted taking his food and that was the cause of his suffering from pain, and advised him to say Bismillah again when he restarted his food after talking.
According to Hazrat Ali (a.s.), one should remember God while eating and should not talk too much as the food is a blessing of God and when one consumes that blessing, then it is time to praise and thank God.