Tafsir of Az-Zukhruf 43:21

"Or have We given them a book..."

The pronoun in "before it" refers to either the Qur'an or the Messenger. The meaning is: they attributed the worship of others besides Allah to the will of Allah—a statement they made without any basis in knowledge.

Then He says: "Or have We given them a book before this book" in which We attributed disbelief and abominations to Ourselves, such that they obtained knowledge of that through revelation, and thus held fast to that book and used it as an argument?

Rather, they have no argument to hold onto except their saying: "Nay, they said, 'Indeed, we found our fathers...'" upon a religion (dīn).

It is also recited as ‘alā ummah (with a kasra). Both readings derive from al-amm, which means "to intend." Thus, ummah is the path that is intended—that is, aimed for—just as riḥlah (journey) is for the place to which one travels. Ummah also refers to the state in which the āmm (the one intending) finds himself. It is also said to mean a blessing or a good state.

In the phrase "upon an ummah and indeed we," the latter is the predicate of inna. Alternatively, the prepositional phrase is connected to muhtadūn (guided).

"And similarly, We did not send before you any warner into a city except that its affluent said, 'Indeed, we found our fathers upon a religion, and we are, in their footsteps, following.'"