It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When one of you fasts a day and forgets, and eats and drinks, let him complete his fast, for it is only Allah who has fed him and given him to drink."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8072