Shama-il Tirmidhi Chapter 27, Hadith Number 2 (180).

Aayeshah radiyallahu ‘Anha says: “Rasulullah Sallallahli ‘Alayi Wasallam said: ‘When one eats and has forgotten to recite ‘Bismillah’, then when one remembers while eating, one should recite ‘Bismillahi awwalahu wa aa-khirahu'”.