Ata' narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that whoever forgets to say 'Bismillah' (In the name of Allah) while slaughtering should say 'Bismillah' (In the name of Allah) when eating and eat it, and should not leave it for Satan, since he slaughtered it upon the fitrah of Islam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيد والذبائح / 18893