A latecomer stood up after the imam’s closing salam, so another person came and completed the prayer with him, making the latecomer the imam. What is the ruling on their prayer?

Question:
And Honorable Shaykh, may Allah grant you success, a person caught the last rak'ah of the 'Asr prayer and stood up to make it up at the end of the rak'ah. A man from outside came in and joined the ma'mum who was making up his prayer and prayed with him. Is their prayer valid? And is there anything required of them?

Answer:
The prayer is valid, if Allah wills, but this is a mistake. You joined an imam who was a latecomer, because this latecomer is a ma'mum, and a ma'mum does not become an imam during the prayer unless the imam appoints him as a substitute, the issue of substitution. As for a ma'mum becoming an imam without the issue of substitution, there is no evidence for that. Yes.