Abu Dawud

619. Commentary:
Here, by expressing physical heaviness, the Prophet sallallahu alayhi wa sallam means moderation and balance in the performance of the acts of prayer. That is, I am not able to move quickly into bowing (ruku‘) or to rise from bowing, so do not try to precede me by hastening, but rather perform all the acts only after me.

Benefits and Issues:
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When the follower (muqtadi) goes into bowing (ruku‘) after the imam, then upon raising his head as well, he will be just as much behind the imam. In this way, the follower’s bowing (ruku‘) will also become just as lengthy as the imam’s bowing. The same condition applies to standing after bowing (qawmah), prostration (sajdah), and sitting between the two prostrations (jalsah).

➋
Since bowing (ruku‘), prostration (sajdah), standing after bowing (qawmah), and sitting between the two prostrations (jalsah) are such pillars in which the remembrance of Allah is made, and supplications and glorifications (tasbihat) are recited, therefore, the one who raises his head after the imam does not have to fear, in the case of an imam who leads the prayer according to the Sunnah, that the imam will finish the standing or sitting before he himself rises. The follower of an imam who performs the prayer with proper composure in the pillars (ta‘dil al-arkan), even though he is behind the imam, still joins the imam in the pillars. The meaning of the hadith is precisely this: that even though you bow and prostrate after me, you will still be included with me in all the pillars, so there is no benefit in hastening.