Is prayer valid when performed in underwear that has been soiled with madhy (pre-seminal fluid), and the stains remain after repeated washing? If it is not valid, is it obligatory to dispose of them?

It is sufficient to sprinkle water on clothes if they are affected by madhy (pre-seminal fluid), and this is the correct opinion of scholars. After sprinkling, the garment is considered pure, and one should not pay attention to the whispers of Satan. Madhy is a thin, white fluid that leaves no traces after washing, and it appears that the questioner is suffering from waswasah (whispers/obsessive thoughts) regarding this matter. Therefore, he should sprinkle water on his clothes and not pay attention to anything else.