If anyone forgets that he is fasting and eats or drinks he should complete his fast, for it is only Allah Who has fed him and given him drink.
Sahih Muslim Book 13, Hadith 222
← Previous Hadith The Apostle of Allah (ﷺ) came to me one day and said: Is there anything with you (to eat)? I said: No. Thereupon he said: I shall then be fasting. Then he came to us another day and we said: Messenger of Allah, hais has been offered to us as a gift. Thereupon he said: Show that to me; I had been fasting since morning. He then ate it.

Sahih Muslim Book 13, Hadith 221 Next Hadith → I said to'A'isha (Allah be pleased with her): Did the Messenger of Allah (ﷺ) fast for a full month besides Ramadan? She said: I do not know of any month in which he fasted throughout, but that of the month of Ramadan and (the month) in which he did not fast at all, till he ran the course of his life.

Sahih Muslim Book 13, Hadith 223