It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): "When a Muslim slaughters and forgets to say 'Bismillah' (In the name of Allah) and 'Allahu akbar' (Allah is the Greatest), let him eat the slaughtered animal, for in the Muslim there is one of the names of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيد والذبائح / 18892