If you forget to say “Bismillah” before eating and remember while you are still eating, say:
بِسْمِ اللَّهِ أَوَّلَهُ وَآخِرَهُ
“Bismillāhi awwalahu wa ākhirahu.”
“In the name of Allah at its beginning and at its end.”
The Messenger of Allah [ﷺ] said:
«إِذَا أَكَلَ أَحَدُكُمْ طَعَامًا فَلْيَقُلْ بِسْمِ اللَّهِ، فَإِنْ نَسِيَ فِي أَوَّلِهِ فَلْيَقُلْ بِسْمِ اللَّهِ فِي أَوَّلِهِ وَآخِرِهِ»
“When one of you eats food, let him say, ‘In the name of Allah.’ If he forgets to say it at the beginning, then let him say, ‘In the name of Allah at its beginning and its end.’”
Sunan Abi Dawud 3767
Grade: Ṣaḥīḥ (Al-Albānī)