They said: The evidence for this is that the sunnah establishes the validity of a person changing from praying alone to leading the prayer, as will come later, Allah willing (1). This shows that such a change has no effect. Thus, just as it is valid to change from praying alone to leading the prayer, it is valid to change from praying alone to following an imam; there is no difference. The only distinction is that, in the first case, he becomes an imam, whereas in the second, he becomes a follower.

If someone were to ask: According to the view that this is valid, if he has already prayed part of the prayer, and these people arrive, for example, to perform the Zuhr prayer in congregation, and he had prayed two rak'ahs before they arrived, then joins them when they arrive, his prayer will be complete when they have prayed two rak'ahs. What should he do?

The answer is that he should sit and not follow the imam, because if he were to follow the imam, he would have to pray six rak'ahs, and this is not permissible. So he should sit and wait for the imam, then conclude the prayer with him. Or, if he wishes, he may intend to pray separately and conclude the prayer; he has the choice.

The second type is changing from praying alone to leading the prayer. The author mentioned it in his statement: “such as intending to lead an obligatory prayer”—that is, just as it is not valid for one praying alone to change into an imam during an obligatory prayer.

An example is a man who begins the prayer alone, then one or more people arrive and say, “Lead us in prayer.” He then intends to be their imam. He has thus changed from praying alone to leading the prayer, and this is not valid, because he has changed from one intention to another. The prayer is therefore invalid, just as it would be if he changed from one obligatory prayer to another.