What is the niyyah of fasting? How should one make niyyah of fasting?
It is not necessary for a person to pass the niyyah for fasting through his mind or to say that he would be fasting on the following day. In fact, it is sufficient for him to decide that in obedience to the command of Allah he will not perform from the time of Adhan for Fajr prayers up to Maghrib, any act which may invalidate the fast. If a person forgot that it was the month of Ramadhan, and takes notice of this before Zuhr and if he has not performed some act which will invalidates a fast and he makes niyyah, his fast is valid. But if he takes notice of this after Zuhr, he should not perform any act till Maghrib which invalidates a fast and should, as an obligatory precaution, also observe qadha of that fast after Ramadhan.