The underlying reason is what has preceded: he is unable to perform an essential pillar, so his state is deficient in comparison to one able to perform it, even though his own prayer is valid. The author then makes an exception, saying:
“Except for the imam of a district whose excuse is expected to cease: they pray seated behind him, as a recommendation.”
His statement, “Except for the imam of a district,” is an exception to the last case mentioned, namely, “or stand.”
“Except for the imam of a district” means the regular imam of the mosque.
A district, plural districts, refers to residential quarters and neighborhoods. Thus, if this mosque has a regular imam who is unable to stand, he may lead the people of the district who are able to stand—but on a condition clarified by the author in his statement:
“Whose excuse is expected to cease,” meaning that his inability to stand must be temporary and expected to end. This differs from one whose inability to stand is ongoing, such as an elderly man; prayer behind him is not valid.
In summary, by these words the author—may Allah have mercy on him—has informed us that whoever is unable to perform the essential pillar of standing, sitting, bowing, or prostrating cannot validly lead anyone except one like himself. Standing is the exception: one unable to stand may validly lead those able to stand, subject to two conditions:
- The one unable to stand must be the imam of the district.
- His excuse must be expected to cease—for example, if he develops pain in his back or knee that is expected to go away. In that case, he may lead the people of the district even though he is unable to stand.
His statement, “and they pray,” refers to the people of the district.
His statement, “behind him,” means behind the seated imam of the district.