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.)
Source: Abu Dawud 3/347, At-Tirmizi 4/288.