Does the prayer of a congregant become void if he lags behind the Imam by a period of two or three tasbihs, or if he raises his hands for the takbir before the Imam finishes reciting, or what is the duration by which, if the congregant lags behind the Imam, his prayer becomes void?

If the follower lags behind his Imam by a slight delay (the duration of two or three Tasbihs) due to the Imam's speed or a legitimate excuse (such as difficulty keeping up with a fast Imam), the prayer is not invalidated. The follower's prayer is invalidated if he lags behind his Imam by two or more integral parts (rukns), or by one complete integral part without a valid excuse. Lagging behind by one integral part without an excuse does not invalidate the prayer according to the well-known and sound opinion. Lagging behind by one complete integral part means that the Imam finishes an integral part while the follower is still in the preceding integral part. The prayer is not invalidated if the follower raises his hands for the Takbir before the Imam finishes speaking, even though it is preferable not to precede him.