Reference : Hisn al-Muslim 178

إِذَا أَكَلَ أَحَدُكُمْ طَعَاماً فَلْيَقُلْ 
بِسْمِ اللَّهِ،
فَإِنْ نَسِيَ فِي أَوَّلِهِ فَلْيَقُلْ 
بسمِ اللَّهِ فِي أَوَّلِهِ وَآخِرِهِ
When anyone of you begins eating, say:
Bismillāh.
And if you forget then when you remember, say:
Bismillāhi fī awwalihi wa ākhirih.


When anyone of you begins eating, say:

With the Name of Allah.

And if you forget then, when you remember, say:

With the Name of Allah, in the beginning, and in the end.

Reference:
Abu Dawud 3/347, At-Tirmidhi 4/288. See Al- Albani's Sahih At-Tirmidhi 
2/167.

Chapter: Before eating