Should one who is observing a voluntary fast and drinks forgetfully before noon complete their fast or break it?

A fasting person who eats forgetfully has a valid fast and must complete it, whether the fast is obligatory or supererogatory, according to the saying of the Messenger of Allah (peace and blessings be upon him): "Whoever eats forgetfully while he is fasting, let him complete his fast, for it is Allah who has fed him and given him drink."