((إِذَا أَكَلَ أَحَدُكُمْ طَعَاماً فَلْيَقُلْ بِسْمِ اللَّهِ، فَإِنْ نَسِيَ فِي أَوَّلِهِ فَلْيَقُلْ بسمِ اللَّهِ فِي أَوَّلِهِ وَآخِرِهِ)).
(When you are about to eat, you should say:(Bismil-lah.)and if you forget to say it before starting, then you should say (when you remember):In the name of Allah in it’s beginning and end.)
Abu Dawud 3/347, At-Tirmidhi 4/288. See Al- Albani's Sahih At-Tirmidhi 2/167.