The one who joins the prayer late when the imam adds a rak'ah by mistake
Machine translationRead the Arabic original
Question:
If a person joins a congregation that is praying and he has missed a rak'ah, and the imam adds a rak'ah by mistake, does he say the taslim with the imam, or does he perform a rak'ah?
Answer:
He must perform a rak'ah and not count it, because this is not part of the prayer; it is extra, so the one who joined late does not count it. Yes.
If a person joins a congregation that is praying and he has missed a rak'ah, and the imam adds a rak'ah by mistake, does he say the taslim with the imam, or does he perform a rak'ah?
Answer:
He must perform a rak'ah and not count it, because this is not part of the prayer; it is extra, so the one who joined late does not count it. Yes.
Source
نور على الدرب - الفوزان - 03/214 — archive.org (13:44)
The Arabic text is a machine transcription of the audio recording.