Is the imam's prayer valid if he prays with only one person behind him?
Machine translationRead the Arabic original
Question : What about the imam?
Answer : There is a distinction here. If he remains with the intention of leading the prayer, his prayer is not valid, because he intended to lead but no one is with him. If he prays alone, his prayer is valid. As for the follower, his prayer is not valid. Where would he stand? To the imam's right, either as an obligation or as a recommended practice, depending on the disagreement you have heard.
Answer : There is a distinction here. If he remains with the intention of leading the prayer, his prayer is not valid, because he intended to lead but no one is with him. If he prays alone, his prayer is valid. As for the follower, his prayer is not valid. Where would he stand? To the imam's right, either as an obligation or as a recommended practice, depending on the disagreement you have heard.
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، 3375هل تصح صلاة الإمام إذا صلى خلفه الفذ؟📔 الشرح الممتع على زاد المستقنع - ابن عثيمين — youtube.com
The Arabic text is a machine transcription of the audio recording.