What is the ruling on the prayer of a latecomer who joined the Imam during the first Tashahhud, said the opening Takbir (Takbirat al-Ihram) and another Takbir, and remained standing until the Imam stood for the third Rak'ah?

Whoever joins the Imam in prayer is obliged to follow him in sitting for the first tashahhud. Whoever makes the takbirat al-ihram with the Imam and remains standing, not sitting with him, has acted contrary to the rules. If he was ignorant of this, his prayer is invalidated, and he must repeat it as a precautionary measure. However, if he acted intentionally, meaning he intentionally stood up from the first tashahhud and preceded the Imam, his prayer is not invalidated. The takbir that a person pronounces after the takbirat al-ihram in an improper place does not invalidate the prayer, even though some scholars say that sujud al-sahw (prostration of forgetfulness) is obligatory for it.