The ruling on one who forgets the takbir of ihram

This question has not occurred, but a discussion among a group led me to learn the truth, which is: A person came by a path and found a congregation that had prepared for prayer, and he entered with the intention of praying with them, but he forgot to say the takbir of ihram, and he did not remember it until the third rak'ah. Is his prayer valid or invalid because he forgot a pillar? And does he begin the prayer from the bowing in the first rak'ah, or does he begin it when he remembers that he did not say the takbir of ihram?

A: The prayer mentioned is not valid; because he did not perform the takbir of ihram, which is a pillar by the consensus of the Muslims, and the prayer does not become established except with it.