What is the ruling on the prayer of the doctor, the Imam, and the congregants if the Imam enters and displaces the doctor who is leading the people in prayer before the bowing (rukūʿ) and then completes the prayer himself, and what if the Imam and the doctor entered into an advanced rak'ah?

There are four possibilities for the prayer after the departure of the first Imam:

1. If the second Imam and the congregants restart the prayer anew: The prayer of everyone is valid, but interrupting an obligatory prayer is forbidden except for a legitimate excuse. 2. If the second Imam restarts the prayer anew and the congregants build upon their first prayer: The prayer of the Imam is valid, but the prayer of the congregants is invalid because they preceded the Imam with the opening takbir (Takbirat al-Ihram). 3. If the second Imam builds upon the prayer of his predecessor and the congregants restart the prayer: The prayer of the congregants is invalid, and the prayer of the Imam is valid if he built upon the first or third raka'ah in a four-rak'ah prayer. 4. If the second Imam and the congregants build upon the prayer of the first Imam: The prayer of the congregants is invalid, and the prayer of the Imam is valid if he built upon the first or third raka'ah in a four-rak'ah prayer.

Some scholars hold the view that the prayer of everyone is valid if they build upon the prayer of the first Imam. This is a considered opinion and the new Imam might have adopted it.