What is the ruling on the prayer of someone who omits the takbir of rising from the first tashahhud, and how does the latecomer perform it in his prayer?

The prayer is valid, according to the majority of scholars, for whoever omits the takbir (pronouncement of "Allahu Akbar") when rising from the first tashahhud, whether intentionally or by oversight. This is contrary to the Hanbalis, who consider the prayer invalid if it is omitted intentionally, and require the prostration of forgetfulness (sujud as-sahw) if omitted by oversight.

As for the ma'mum (follower), there is no prostration of forgetfulness upon him if he makes an oversight without his imam. However, if a latecomer (masbuq) catches the imam in the first tashahhud, he should only say the takbiratul ihram (opening takbir), then sit down, and then say the takbir when the imam stands for the third rak'ah, in accordance with him.