The answer is that the Prophet, peace and blessings be upon him, said: “When he bows, then bow; and when he prostrates, then prostrate.” (1) He used the particle fa, which indicates sequence and immediate succession without delay. This shows that it is better for the one praying behind the imam not to lag behind him, even slightly; rather, he should hasten to follow him. He should neither move simultaneously with him, nor precede him, nor lag behind him. This is the true meaning of following the imam.
If the situation is reversed—meaning that the imam holds this sitting to be prescribed while you do not—then you must sit. For if you do not sit, you will rise before your imam, and this is preceding the imam, which is forbidden. The Prophet, peace and blessings be upon him, said: “Does the one who raises his head before the imam not fear that Allah may turn his head into that of a donkey, or make his form like that of a donkey?” (2)
He may say: I will not rise before him; rather, I will remain in prostration until I think that he has risen. We say: In that case, you have not done anything prohibited, but you have abandoned a sunnah—namely, hastening to follow the imam. Therefore, if you do not consider it recommended while the imam does, then sit with your imam, just as you sit with him for the tashahhud that does not occur at the proper point for your own tashahhud, in order to follow him.
“And he performs the second rak'ah in the same manner, …………”
His statement, “And he performs the second rak'ah in the same manner,” means that he performs the second rak'ah like the first. Accordingly, “second” is an adjective whose noun has been omitted; the intended meaning is: like the second rak'ah.