so if the Lawgiver were, for example, to say, “Do not pray in this garment,” and he prayed in it, we would say: The prayer is invalid if you pray in this garment, because praying in it contradicts the Lawgiver’s prohibition of praying in it. But since the Lawgiver has not prohibited praying in this garment—rather, He has prohibited wearing an unlawful garment absolutely, whether in prayer or otherwise—this does not entail that the prayer is invalid, because the two considerations are separate. The command to wear a garment in prayer is due to the prayer itself: “O children of Adam, take your adornment at every place of worship.” [al-A‘raf: 31] The prohibition against wearing an unlawful garment, however, is not due to the prayer, but because it entails using something that you are not permitted to use. ...
This view—namely, that the prayer is valid when one covers his private parts with an unlawful garment—is the stronger view, unless the hadith concerning the one who lets his garment hang below his ankles, requiring him to repeat the prayer, is established as authentic. If the hadith is established, then it becomes necessary to hold to what it indicates. However, many scholars deemed it weak (1), saying that it does not constitute proof; and one cannot require someone to repeat his prayer on the basis of a weak hadith.
What if he prays in an unlawful garment while wearing another garment as well? The apparent meaning of the author’s words is that the prayer is invalid, because he said, “or prays in a garment unlawful for him,” and did not say, “covers himself with a garment unlawful for him.” Accordingly, if he prays in a silk garment with a cotton or wool garment beneath it, then his prayer is invalid according to the implication of the author’s words. It was also said: if the unlawful garment is the inner garment and the permissible one is the outer garment, the prayer is invalid; but if the reverse is the case, it is valid (2).
The inner garment is that which touches the body, while the outer garment is the one worn over it, because if it is