"When you are about to eat, you should say:
In the Name of Allah.
...and if you forget to say it before starting, then you should say [when you remember] :
بِسْمِ اللَّهِ فِي أَوَّلِهِ وَآخِرِهِ
In the Name of Allah, in the beginning and in the end.
Bismillaahifee 'awwalihi wa 'aakhirihi.
Abu Dawud 3/347, At-Tirmithi 4/288. See Al- Albani's Sahih At-Tirmithi 2/167.