The right of a person other than the imam to lead the prayer

Question

Does an imam who is appointed to a mosque have, according to the Shari‘a, the full right to permit or not permit anyone to stand on the imam’s place?

Answer

In the presence of the appointed imam, no other person is permitted to go to the imam’s place and lead the prayer. Therefore, if someone arrives at a mosque that has a regular (daily) imam, he should pray behind that regular imam. However, if [the imam] himself asks him to lead, then he may lead the prayer.

Source: Fatawa Mahmudiyya

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