What is the ruling on someone who was fasting to make up for missed days and drank water forgetfully, then remembered before swallowing it but proceeded to swallow it? Does he owe expiation?

Whoever eats or drinks forgetfully while fasting, their fast is valid and not broken, whether it is an obligatory or a supererogatory fast. This is based on the Prophet's (peace be upon him) saying: "Whoever forgets while he is fasting and eats or drinks, let him complete his fast, for it was Allah who fed him and gave him drink." However, if one remembers while eating or drinking, they must spit out what is in their mouth and not swallow it. If they swallow it after remembering, their fast is broken, and they must repent and make up that day without expiation.