Al-ʿAllāmah Muḥammad ibn Ṣālih al-Uthaymīn [d. 1421 AH] said [concerning doubts about clothing that may have najāsah]: The correct opinion is that an individual should seek out the truth with regards to it. If it seems most probable to him that a piece of clothing is pure, then he should pray in it. For Allāh does not burden a soul more than it can bear, neither has He made it obligatory on anyone that he should pray a prayer twice [one in each piece of clothing he has doubts concerning].
If you say: Is it not possible that even with this inspection [of the clothing] that he could hypothetically have prayed in something that has najs on it?
We answer: Of course, but this is the extent of his ability. Also, praying in clothing that has najs is permissible in extreme conditions according to the correct opinion. As for the [Ḥanbalī] madh`hab, their opinion is that one should pray in it [the first doubtful garment] and then repeat their ṣalah [with their second doubtful garment]. So, for example, if a man was in the desert and had only one piece of clothing that had najs and he lacked anything to purify it, he would have had to pray in it. Even if it is for an entire month. Then, if he were to purify that piece of clothing he would then have to re-pray all of his ṣalah for that entire month. He is praying because the time has come and he has been commanded with the prayer. He is repeating his prayer because he has prayed in clothing that has najs. This is a weak opinion. What is correct is that he prays and does not repeat his ṣalāh. As they [the Ḥanbalī scholars] will also say if someone has to pray ṣalāt al-khawf [in wartime] and he is forced to pray it carrying his weapon that has najs on it, he should pray with it on his person and he doesn’t need to repeat his ṣalāh. Likewise we say, this is also an extreme situation. Otherwise, what would you have [this soldier] do?”
Source: Al-Sharḥ al-Mumtiʿ: 65-66
Translated by: Riyāḍ al-Kanadī