What is (incumbent) upon someone who prayed while his garment had slight impurity (such as feces), in two scenarios: if he was unaware of it at the time of the prayer then came to know of it, or if he was aware of it but forgot to remove it and prayed then remembered?

Purity from impurity on the body and clothes is a condition for the validity of prayer according to the majority of scholars. Among their proofs is Allah's saying: "And purify your garments."

As for one who prays with an unforgivable impurity on his clothes: - If he prays while aware of it, his prayer is invalid and he must repeat it. - If he prays while unaware of the impurity until he finishes the prayer, there are three opinions on this matter: 1. His prayer is not invalidated (this is the most preponderant view). 2. He must repeat the prayer. 3. He must repeat what was within the prayer's prescribed time but not afterward.

As for one who knew of the impurity's presence and intended to remove it, but then forgot and prayed, and remembered afterward, he is not required to repeat the prayer according to the soundest of the two scholarly opinions.