Tafsir of Al-Muddathir 74:37

The prepositional phrase acts as a substitute for the preceding prepositional phrase—namely, "to mankind" (li-l-bashar). The pronoun in "willed" (sha'a) refers to the relative pronoun, meaning: a warner for those among you who are capable of hastening toward good or lagging behind it.

Al-Suddi said: "To advance" means to move toward the Fire mentioned previously, or "to lag behind" means to move away from it toward Paradise. Al-Zajjaj said: "To advance" means toward what is commanded, or "to lag behind" means away from what is prohibited. Some have interpreted "advancing" as faith and "lagging" as disbelief.

It has also been said that the pronoun in "willed" refers to Allah the Exalted, meaning: a warner for whomsoever Allah, the Exalted, has willed among you to advance or to lag behind.

It is permissible for "to whom" (li-man) to be a fronted predicate, and "to advance or to lag behind" to be the subject, as in your saying: "For whoever performs ablution, it is [permissible] to pray." Its meaning is absolute: for whoever wills to advance—that is, to hasten toward good—or to lag—that is, to delay from it—[the choice] to advance or to lag behind is his. Thus, it would be like His saying, the Exalted: "So whoever wills, let him believe; and whoever wills, let him disbelieve." It is not hidden that the phrasing admits this, but it is very far-fetched.