What is the ruling on one who did not prostrate with the Imam because the Imam did not say the Takbir for prostration, and he only prostrated after the Imam said the Takbir to rise from prostration?

If the worshiper realizes the imam's prostration while remaining standing until the imam rises from prostration, his prayer is invalid according to some scholars. However, if he prostrates after the imam and prays, his prayer may be valid. Some scholars are of the view that if the worshiper does not notice the imam's prostration until after he has risen, then his delay from the imam in this instance is excused, and he must prostrate and then follow his imam, and his prayer will be valid.

Shaykh Ibn Uthaymeen divides delay from the imam into two types:

1. Delay with an excuse: The worshiper performs what he missed and follows the imam. If the imam reaches his position (in the next rak'ah), he should not perform the missed pillar, and he will have a composite rak'ah.

2. Delay without an excuse: Delay within a pillar: This is when you delay in following but catch up with the imam in the pillar he has moved to. Your prayer is valid, but the action is contrary to the Sunnah (definition). Delay by a full pillar: This is when the imam precedes you by a complete pillar. Your prayer is invalid, whether the pillar was a bowing (ruku') or anything else, because this is not considered following.