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'". (ES)
English reference (ES): Book 27, Hadith 180 (ES)
Arabic reference (ES): Book 28, Hadith 189 (ES)