It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that, regarding a person who forgot to say 'Bismillah' (with the name of Allah) at the time of slaughter, he said: The name of Allah is within the Muslim, even if he did not say 'Bismillah' (with the name of Allah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيد والذبائح / 18891