The imam stood for the third rak'ah and forgot the tashahhud, so some of the ma'mum sat down; when the imam said takbir for ruku', they stood up and bowed with him.

Question:
Honorable Shaykh, may Allah grant you success. The questioner says: We prayed the Zuhr prayer today, and the imam forgot to sit for the first tashahhud. He had already stood up fully, so he did not go back. Some of the people sat down waiting for him to sit, until the imam said takbir for ruku'. They stood up and bowed with him, and they did not recite al-Fatihah. What is the ruling on their prayer?

Answer:
Their prayer is valid because they did not sit down intentionally; rather, they sat down out of ignorance. So their prayer is valid, if Allah wills, and the imam's recitation of al-Fatihah is sufficient for them. Yes.