Nor does a mujtahid (independent jurist) defer to another mujtahid whose conclusion contradicts his own, nor may he pray behind him; a muqallid (lay follower), however, follows the one he regards as the more reliable.

Anyone who prays without either ijtihād or taqlīd while capable of one of them must repeat the prayer; if incapable, he does his best to determine the qiblah and then prays.

A qualified person makes ijtihād for every prayer; if a second ijtihād yields a different result, he acts on the second and is not required to make up what he performed according to the first.

Among the prerequisites is the intention: he must specify the very prayer he is performing—such as Zuhr or a regular Sunnah prayer (1)—and need not articulate that it is obligatory, on time, or otherwise.

The intention must coincide with the opening takbīr or be formed a brief moment before it while the time remains; should he abandon it or waver about it (2), the prayer is invalid.

It is permissible to convert an obligatory prayer into a voluntary one so long as the time is still ample, though doing so without a legitimate purpose is disliked.

Both imam and follower must intend their respective roles; if someone praying alone intends to lead or to follow, the prayer is not valid.

A follower’s prayer is nullified if he leaves the congregation without an excuse that legally permits missing the communal prayer, and it is likewise nullified by the imam’s prayer becoming invalid.

The imam may appoint a deputy because of illness or an inability to perform an essential act, and the deputy continues the prayer from the point where the imam stopped.

If the regular imam begins the prayer with the very congregation for whom his deputy had already commenced it, and the deputy then joins as a follower, the arrangement is valid.