Q. Is the [general] intention of fasting sufficient for intending to fast every single day?
A. It is well-known that every person wakes up at the end of the night and takes their pre-dawn meal as they no doubt want to fast. This is because every mentally discerning person does something by their own choice – it is not possible to do something without wanting to. This ‘wanting [to do something]’ is the intention, and so no person would eat at the end of the night except for the sake of fasting. Even if one wants to merely eat, it would be [extremely] unusual for them to eat at this hour. Thus, this is the intention.
Yet, similar to this question, one needs to know that if a person was to sleep before sunset in Ramaḍān and remained asleep, while no one woke them up, and then they awoke the following day at Fajr time, while they still have not intended from the [previous] night to fast the following day, then do we say: their fasting for the following day is acceptable on the basis of his previous intention, or do we say that their fasting is unacceptable because they have not intended it from the night [before]?
We say: their fasting is acceptable because the most preferred position is that the intention to fast [the whole of] Ramaḍān in its beginning is sufficient and does not require a renewal of intention every day. Unless, if there is a reason that permits a person to break their fast, and they break their fast during Ramaḍān, then in that case they would be required to renew their intention for fasting.
Shaykh Ibn ʿUthaymīn – Majmūʿ al-Fatāwā wa Rasāʾil (176/19)