Lagging behind by an entire pillar means that the imam precedes you by a pillar; that is, he bows and rises before you have bowed. The jurists—may Allah have mercy on them—say that lagging behind is like preceding the imam: if you lag behind in bowing, your prayer is invalid, just as it would be if you preceded him in it. But if you lag behind in prostration, then, according to the jurists, your prayer is valid, because this is lagging behind in a pillar other than bowing.

However, the sounder view, in accordance with what we have regarded as preponderant concerning preceding the imam, is that if one lags behind him by an entire pillar without an excuse, his prayer is invalid, whether that pillar is bowing or something else. Accordingly, if the imam rises from the first prostration while this follower is supplicating Allah in prostration, and he remains there supplicating until the imam performs the second prostration, his prayer is invalid, because he has lagged behind by a pillar. If the imam has preceded him by a pillar, then where is the following?

Third: moving in unison.

Moving in unison may be in verbal utterances or in actions, and is of two types:

The first type is moving in unison in verbal utterances. This does not cause any problem except in the opening takbir and the salām.

As for the opening takbir, if you say the takbir before the imam has completed his opening takbir, your prayer has not even begun validly. You must pronounce the opening takbir only after the imam has fully completed it.

As for moving in unison with the imam in the salām, the scholars say that it is disliked to say the first and second salāms together with him. However, if you say the first salām after his first salām, and the second after his second, there is nothing wrong with this. Yet it is better not to say the salām until after he has completed both salāms.