What does one do if he enters the mosque and finds the imam in prostration? Does he wait until he stands for a new rak'ah, or does he join him in his prostration?

The student : ... regarding the prostration, he is in prostration and you are standing, so you miss the reward of prostration. Prostration entails a great reward, especially the supplication. Then, may Allah know best, it seems Mu'adh established this sunnah for them, so he said, meaning the people of the Sunnah, "Mu'adh has established a sunnah for you, so follow it," may Allah know best.

The Shaykh : This is the final answer.
The student : Yes, this is the final answer.

The Shaykh : This is the correct view. As for what you presented earlier, it is subject to scrutiny, especially when you cited as evidence his saying (peace be upon him): . This is valid evidence for one who has already joined the prayer, but the assumption here is that he has not joined the prayer yet. Is that clear?
The student : Has he not joined the prayer yet?

The Shaykh : This is the one who came, entered, and found the imam in prostration; he has not joined the prayer yet.
The student : Fine, he says the takbir and joins him.

The Shaykh : Be patient. May Allah improve my condition and yours. Say, "Ameen."
The student : Ameen, O Allah.

The Shaykh : This is what you are asking about: he entered the mosque and the imam was in prostration. So the question is, what does he do? Does he join him or not? You are citing the Messenger's saying as evidence that he should join him: . He has not become his imam yet, so it is not valid to cite this hadith for the situation you are in. The strongest thing you mentioned earlier was what you pointed out regarding the virtue of prostration: . This evidence is correct, and stronger than it is the story of Mu'adh ibn Jabal, where he (peace be upon him) said: . He found the imam in prostration and prostrated, so this is evidence.
The student : When Mu'adh prostrated, did the Messenger (peace and blessings be upon him) know about it? Or how did the Messenger know that Mu'adh prostrated...?

The Shaykh : That does not matter. The answer to this is either that Allah informed him of it, or that it is due to his saying (peace be upon him): . This is a special attribute of his.