bear from them except one child, half-formed.” The Prophet ﷺ said: “Had he said, ‘If Allah wills,’ it would have enabled him to attain his wish.” (1)
However, if he says this while uncertain—that is, not knowing whether he will fast or not—then it is invalid, because the intention must be definite. Thus, if he goes to sleep with this intention, saying, “I will fast tomorrow, if Allah wills,” while uncertain, his fast is invalid if it is obligatory—unless he wakes before dawn and forms the intention then.
Al-Rawd states: “Eating and drinking with the intention of fasting suffice as an intention.” (2) That is, if he rises at the end of the night and eats, considering it to be the pre-dawn meal, that suffices. Shaykh al-Islam even said: The evening meal of one who will fast the next day differs from that of one who will not fast the next day. The one who will not fast eats more at his evening meal, because the fasting person will leave room for the pre-dawn meal.
A voluntary fast is valid with an intention formed during the day, before or after midday, ………
His statement, “A voluntary fast is valid with an intention formed during the day, before or after midday,” means that a voluntary fast is valid when its intention is formed during the day, whether before or after midday. This contrasts with his statement, “The intention must be specified at night for the fast of every obligatory day.” A voluntary fast is therefore valid with an intention made during the day, on condition that one has not done anything that invalidates the fast after the break of dawn. If he has done something that invalidates the fast, it is not valid.
An example is a man who begins the day and decides to fast during the day, having neither eaten, drunk, had intercourse, nor done anything that invalidates the fast after dawn. His fast is valid, even though he had not formed the intention before dawn.