If the congregation has finished, is it prescribed for him to follow one of the ma'mums, making him his imam?
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Question:
He says: Honorable Shaykh, may Allah grant you success. If a worshipper enters and finds that the imam has finished his prayer, and some of the ma'mums stand up to complete what they missed, is it prescribed for the one who entered to stand with that ma'mum?
Answer:
Yes, the masbuq. Yes, it is not better. Some scholars permit this, but the correct view is that it is not permissible, because he is a ma'mum, and a ma'mum does not become an imam unless the imam appoints him as a deputy for a reason. Yes.
He says: Honorable Shaykh, may Allah grant you success. If a worshipper enters and finds that the imam has finished his prayer, and some of the ma'mums stand up to complete what they missed, is it prescribed for the one who entered to stand with that ma'mum?
Answer:
Yes, the masbuq. Yes, it is not better. Some scholars permit this, but the correct view is that it is not permissible, because he is a ma'mum, and a ma'mum does not become an imam unless the imam appoints him as a deputy for a reason. Yes.
Source
قناة «العلامة الدكتور صالح الفوزان»، [433 -521] إذا وجد الجماعة قد انقضت، هل يشرع له أن يأتم مع أحد المأمومين فيجعله إماما له؟ — youtube.com
The Arabic text is a machine transcription of the audio recording.