Tafsir of Al-Hujurat 49:3

{God has tested their hearts for piety} This is derived from your saying: "He tested so-and-so for such-and-such," meaning he tried him and trained him to rise to the task. He is thus capable of it, not negligent regarding it. The meaning is that they are patient in piety, strong in bearing its hardships.

Alternatively, "testing" is used in the place of "knowing," because the reality of a thing is verified by testing it—just as "experience" is used in the place of "knowledge." It is as if it were said: "God has known their hearts for piety." In this case, the lam (in li-l-taqwa) is connected to an omitted predicate, and it is the same lam as in your saying: "You are for this matter," meaning you are suited and designated for it. The poet said: You are for it, O Ahmad, among all mankind, Enemies of those who travel on foot over the rugged path. The lam and its object are in the position of an accusative state (hal).

Or, it means: God struck their hearts with various trials and difficult obligations for the sake of piety—that is, so that their piety might be established and made manifest, and it might be known that they are pious. For the reality of piety is not known except during trials, hardships, and patience through them.

It is also said: He purified them for piety, from the saying: "He tested the gold and refined it," meaning he melted it to extract its pure essence from its dross and cleansed it. Regarding this, Umar (may God be pleased with him) said: "He removed desires from them."

Imtihan (testing) is an ifti'al form from mahana, meaning an eloquent test or a strenuous trial. Abu Amr said: "Everything you have exhausted, you have tested." He recited: The exhausted camels arrived, their fatigue apparent, They have been tested, and their flanks have trembled.

It is said this was revealed regarding the two Shaykhs (Abu Bakr and Umar), may God be pleased with them, due to their lowering of their voices and reaching the point of whispering to the Prophet.

This verse, in the structure it is arranged upon—by making those who lower their voices the subject of Inna (the particle of emphasis), making its predicate a sentence composed of a definite subject and a definite predicate (the demonstrative pronoun), initiating the sentence that contains their reward for their deed, and presenting the reward as an indefinite noun to signify the magnitude of its importance—all points to the extreme appreciation and approval of what those who honored the Messenger of God (peace be upon him) did by lowering their voices. It also serves to inform of the extent of the Messenger of God’s honor and the magnitude of his status, while implicitly rebuking the gravity of what those who raised their voices committed, and their deserving of the opposite of what these [pious ones] deserved.

{Indeed, those who call out to you from behind the chambers—most of them do not reason. And if they had been patient until you came out to them, it would have been better for them. And God is Forgiving, Merciful.}