I read that it is not permissible for a follower to become an imam, so a latecomer should not follow a follower who missed a rak'ah or more. What is the guidance?

Question:
He also says: I read in a book on the conditions of congregational prayer that it is not permissible for a follower to be an imam, and it is not permissible for a person outside the prayer to follow someone who missed a rak'ah or more, because he was a follower, so it is not valid for him to be an imam. If one of them becomes a follower, the prayer of the one being followed is invalid. What is your opinion on this matter?

Answer:
Yes, there is discussion among the scholars on this matter. A follower does not become an imam except in the case of appointment (istikhlaf). If the imam is appointed by the imam when something happens to the imam in the prayer that requires him to leave, and he appoints one of the followers, there is no harm in that. Just as 'Umar was appointed when he was stabbed; he appointed 'Abd al-Rahman ibn 'Awf, who prayed the remainder of the prayer with the people and completed the prayer with them. There is no harm in that if the imam appoints someone.
As for if he becomes an imam without being appointed by the imam, this is the point of research and the point of disagreement among the scholars. The better course is to avoid this matter to get out of the disagreement and to be cautious with the religion, so a follower should not be an imam except in the matter of the imam appointing him.