Not being at rest in the sitting between the two prostrations

Question:
Honorable Shaykh, the questioner says: What is the ruling on one who does not settle in the sitting between the two prostrations and does not sit upright until the imam prostrates the second time?

Answer:
Do not delay in prostrating, do not delay in prostrating. When the imam rises, rise with him in order to sit between the two prostrations and say, “My Lord, forgive me.” Do not miss the sitting between the two prostrations; this would be wasting the sitting. The sitting between the two prostrations is obligatory, and saying “My Lord, forgive me” is obligatory, so do not miss this. Continue prostrating, then stand up, and if the imam is prostrating the second time, this is a loss for himself; he has lost for himself two obligatory things: the obligatory sitting and the obligatory saying of “My Lord, forgive me.” Yes.