Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "When one of you forgets and eats or drinks while he is fasting, 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 / 8071
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
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: A man came to the Prophet (peace and blessings be upon him) and said: I ate and drank out of forgetfulness. He (peace and blessings be upon him) said: "Complete your fast, for indeed Allah has fed you and given you to drink."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8073
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "Whoever breaks his fast in Ramadan out of forgetfulness, there is no making up due from him and no expiation." It is reported from Mujahid and al-Hasan (may Allah have mercy on them both), concerning this and concerning intercourse out of forgetfulness, that there is no making up due from him; and Ata' (may Allah have mercy on him) used to say: Making up is obligatory for intercourse out of forgetfulness.
Only al-Ansari narrated it from Muhammad bin Amr, and all of them are thiqat (trustworthy); and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8074