to follow, but the imam does not intend to lead, there are two opinions:

The first is that the prayer is valid. This is the view of al-Shāfiʿī, Mālik and others, and is also one view reported from Ahmad.

The second is that it is not valid. This is the well-known view reported from Ahmad.

The explanation is that this man was following an imam at the beginning of the prayer, then became someone praying alone after the imam’s salutation. When the other man follows him, he changes from praying alone to leading, just as the Prophet, may Allah bless him and grant him peace, became the imam of Ibn ʿAbbās after initially praying alone. This is valid in voluntary prayer, as illustrated by this hadith and explicitly stated by Ahmad and other leading scholars, although an opinion that it is impermissible has been mentioned within Ahmad’s school.

As for obligatory prayer, there is a well-known disagreement. The correct view is that this is permissible in both obligatory and voluntary prayers, since, by leading, the imam assumes more obligations than he had when praying alone. Thus, there is nothing objectionable at all in someone praying alone becoming an imam, unlike the former case. Allah knows best.