Tafsir of Ya seen 36:21

This is a repetition for emphasis, used as a means to reach the description of them with that which entails the negation of any impediment to following them, after having indicated the realization of the requirement.

His saying, Exalted is He: {وَهُم مُّهْتَدُونَ}—meaning they are steadfast upon guidance in what they are upon, regarding the goodness of this world and the Hereafter—is a circumstantial sentence (jumla hali). It contains that which confirms that they neither ask for a wage nor for that which follows it, such as the seeking of status and loftiness. For this reason, it is rendered as an excellent hyperbole (ighal), similar to the saying of Al-Khansa: "And indeed, Sakhr is one whom the guides follow, as if he were a landmark upon whose summit is a fire."

It is apparent that the man did not say this except after his faith had already preceded it. It is narrated that when the call reached him, he came striving, heard their speech, and understood it. He then asked them: "Do you seek a wage for this invitation of yours?" They replied: "No." At that point, he called his people to follow them and believe in them, saying: "{O my people}..." etc.

Regarding this type of construction, the grammarians hold two views:

First, that "who" (man) is a substitute (badal) for "the messengers," with the repetition of the operative word ('amil) just as it is repeated when it is a preposition, such as His saying, Exalted is He: {We would have made for those who disbelieve in the Most Merciful... for their houses...}. Some have adopted this view.

Second, which is the view of the majority, is that it is not a substitute. For the substitute is specific to cases where the repeated operative word is a preposition. As for when it is a nominative or accusative, they term that "follow-up" (tatbi') rather than "substitution" (badal).

The verse is used as evidence for the deficiency of anyone who takes a wage for any act of religious duty, and this discussion is comprehensively addressed in the books of jurisprudence (furu').