Abu Hurairah (May Allah be pleased with him) reported:
The Prophet (ﷺ) said, "If any one of you forgetfully eats or drinks (while observing fasting) he should complete his Saum (fasting), for Allah has fed him and given him to drink."[Al-Bukhari and Muslim].
App reference: Book 9, Hadith 252
Arabic/English book reference: Book 9, Hadith 1242