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