Is it permissible for one who finds the Imam prostrating and wishes to join him in prayer to enter with a single Takbir, or is it necessary for him to pronounce two Takbirs?

The opening takbir (Takbirat al-Ihram) is a pillar of prayer, without which the prayer is not valid. It is obligatory upon the ma'mum (follower) to say it when joining the prayer while the Imam is in prostration. It is sunnah for him to say a second takbir for the prostration, and his prayer is valid even if he does not say the second takbir.