If one catches the imam in prostration, is he required to say only the takbir for prostration, or the takbir of entering prayer and the takbir for prostration?

Question:
He says: Honorable Shaykh, may Allah grant you success, if a person joins the imam while he is in prostration, is he required to say the takbir of entering prayer and the takbir for moving into prostration, or is one takbir sufficient if he intends it to be the takbir for prostration, since the rak'ah has been missed for him?

Answer:
If he omits the takbir for prostration, his prayer is not valid. He must intend the takbir of entering prayer, because it is a pillar of the prayer; its prohibition is established by the takbir, and it is a pillar of the prayer. He does not intend the takbir of entering prayer [to be the takbir for prostration]. As for the second takbir, the takbir for moving into prostration, in this position it is a recommended practice (sunnah); if he wishes, he says it, and if he wishes, he does not. Yes. He also says the takbir of entering prayer while standing. Then he goes down into prostration; he does not say the takbir after he has lowered himself, nor during the lowering. It is not required that he say it while standing. Yes.