—for, when the time of the obligatory prayer becomes tight, that span can accommodate nothing else. If, however, there is still plenty of time left while the congregation has already finished, we say: there is no harm in converting it into an unrestricted voluntary prayer, giving the salām after two rak‘ahs, going to your appointment, and then returning later to perform the obligatory prayer. Thus the possible transitions are three:

1. From an unspecified prayer to a specified one: the specified intention is invalid, whereas the original unspecified prayer remains valid.
2. From one specified prayer to another specified prayer: the first is nullified and the second never takes effect.
3. From a specified prayer to an unspecified one: the conversion is valid, but the specified obligation still rests upon him.

Intending to lead and to follow. A congregational prayer requires both an imam and a follower; the minimum is two people—an imam and a follower—and the larger the congregation, the more beloved it is to Allah. The follower must intend to follow the imam; this is unanimously agreed upon. In other words, when you enter a congregation you must intend to complete your prayer behind the imam with whom you have joined. Yet, as we have said, this intention needs no elaborate effort, for anyone who comes to the mosque has, by that very act, intended to follow.

As for the imam, the scholars—may Allah have mercy on them—differed: must he intend to lead, or is that not necessary? Some maintained that he must. According to this view, if two men arrive, find someone praying alone, and decide to take him as their imam, so they form a row behind him while he is unaware of them, yet they intend him to be their imam and begin following his movements, then those who say