Praise be to Allah.
Shaykh `Abdullah ibn Jibrin (may Allah have mercy on him) said:
It is permissible for one person to take care of reciting the Adhan and leading the prayer. If the Mu’adhdhin has more knowledge of the Quran than anyone else, then he may lead the people present in prayer as the Imam.
The same applies if the usual Imam is absent and appoints him to act in his stead. It is also permissible for him to be appointed as the regular imam. (End quote)
For more details, please see the following answers:
Who has the most right to lead the prayers?
The imam and Mu’adhdhin are alternating in leading the prayer and giving the Adhan
The reward for the Mu’adhdhin and the one who gives the Iqamah
Is it Sunnah for the Mu’adhdhin to pray directly behind the Imam?
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