he forms the intention?

Ahmad’s explicit statement is that the reward begins from the time the intention is formed. They also differed over specifying the type of fast in the intention. There are three views on this within Ahmad’s school and elsewhere:

The first is that one must specifically intend to fast Ramadan. Neither a general intention to fast nor an intention specifying a fast other than Ramadan suffices. This is the view of al-Shāfi‘ī and one of the two positions reported from Ahmad, adopted by many of his followers.

The second is that either a general intention to fast or an intention specifying another fast suffices. This is the position of Abū Hanīfah and a position also reported from Ahmad.

The third is that a general intention to fast suffices, but not an intention to observe a voluntary fast, make up a missed fast, or fulfil a vow. This is a position reported from Ahmad and adopted by a group of his followers.