All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If someone forgetfully eats or drinks while fasting, then he should continue fasting regardless of how much and how many times he eats and drinks. This is based on the general indication of the statement of the Prophet, sallallaahu ‘alayhi wa sallam, who said: “Whoever forgetfully eats or drinks while he is fasting should complete his fast for indeed it is Allaah Who provided him with food and drink.” In this regard, there is no difference whether the offered fast is obligatory or voluntary, given the all-inclusiveness of the statement of the Prophet, sallallaahu ‘alayhi wa sallam.
Allaah Knows best.