Issues related to the intention of fasting

In our religion, the correctness of every action and its acceptance in the sight of Allah depends on the intention. Similarly, making intention is a prerequisite for fasting. Without intention, there is no worship.
The obligatory requirements of fasting during the month of Ramadan are three: intention, abstinence from food and drink from dawn to sunset, and abstinence from sexual intercourse.

In fact, intention is something that is in the heart. Only the intention made in the heart is counted. If the intention is made on the tongue but there is nothing in the heart, the fast is not a fast.

As Mawlana Ali Qari said, the innovation of making intention on the tongue is considered a good deed, that is, an acceptable deed. Accordingly, one makes intention by saying in one's heart and on the tongue, as usual, such as: "I intend to fast the month of Ramadan from dawn to sunset."
In fact, our fiqh books state that making intention with the tongue is also a sunnah ("Alamgiriyya", "Kofi", "Nihaya", "Nahr").

All the jurists of our school have stated that one should make a separate intention for each day of Ramadan ("Fatavoyi Qazikhan"). However, among the Malikis, it is permissible to fast the entire month of Ramadan with one intention.

Imam Najmuddin Nasafi actually said that getting up for sahur in Ramadan counts towards the intention. However, getting up for sahur only counts towards the intention for that day, and it does not count towards other days. The time for the intention starts every day after sunset, and the intention made before that does not count (Sarakhsi, “Muhiyt”).

If a person makes the intention to fast the next day before sunset, but then falls asleep or faints, or for some reason does not have time to renew his intention before dawn, his intention is not invalid. However, if he makes the intention after sunset, it is valid (“Hulosa”). There is no difference between a traveler and a resident in terms of intention (“Tabyin”).

It is preferable to make the intention at the right time, that is, at night. However, it is also permissible to make the intention during the day.

It is also important that the intention is clear, that is, that the intention is clear and firm as to which fast is being intended ("Ikhtiyor").

If a person who oversleeps and cannot get up for sahur wakes up after sunrise, makes the intention to fast without eating or drinking anything, according to our school of thought, his fast for that day is valid. This is also true if he wakes up before sunrise. In both cases, he still makes the intention to fast normally during Ramadan.

However, it is not permissible to do this all the time (i.e., to make the intention to fast after dawn or sunrise). This is because it is not permissible in other schools of thought. Our Prophet, may God bless him and grant him peace, said: “Whoever completes his fast before dawn and does not make the intention, then his fast is invalid.” (Narrated by Abu Dawud and Tirmidhi)

In the case of fasts of qadha and expiation, it is obligatory to make the intention in the evening and to strictly determine the fast to be observed ("Nikaya").

Prepared based on sources.