Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrated: The Prophet (peace and blessings be upon him) was asked about a fasting person who forgetfully eats or drinks something. The Prophet (peace and blessings be upon him) did not order him to make it up, and he said: This is food that Allah Almighty has fed him.
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat. Its chain contains a narrator, Muhammad bin Ubaydullah al-Arzami, and he is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4899