The ma'mum completing his prayer after the imam who forgot has said the taslim
Machine translationRead the Arabic original
Question:
And between the two children, if the imam asked and did not know, and they informed him a little after the taslim, so the imam stood up, but there were some of them who were completing their prayer, so he stood up to complete his prayer without their knowledge, mocking the imam. So does he complete the prayer and observe? Or is it obligatory for him to return to every prayer?
Answer:
He has the choice: if he wishes, he completes his prayer alone because he has separated from the imam, and if he wishes, he joins the imam and completes it with him. There is no harm in that.
And between the two children, if the imam asked and did not know, and they informed him a little after the taslim, so the imam stood up, but there were some of them who were completing their prayer, so he stood up to complete his prayer without their knowledge, mocking the imam. So does he complete the prayer and observe? Or is it obligatory for him to return to every prayer?
Answer:
He has the choice: if he wishes, he completes his prayer alone because he has separated from the imam, and if he wishes, he joins the imam and completes it with him. There is no harm in that.
Source
شرح بلوغ المرام من أدلة الأحكام [ صالح الفوزان ]، تتمة باب سجود السهو وغيره — islamhouse.com (1:01:46)
The Arabic text is a machine transcription of the audio recording.