What is the ruling on the prayer of someone who forgot the منديل (tissue/cloth) that he places in his underwear after istinja' (purification after defecation/urination) to guard against urine drops, and only remembered after the prayer?

One should not pay attention to a mere feeling that some urine remains, for fear of falling into obsessive doubts (waswas). However, if the questioner is certain that urine has exited after purification, then it is permissible to wrap the private part with a cloth to avoid contamination. This cloth should be removed during prayer. If the urine continues to flow, then it is a case of urinary incontinence (sals al-bawl) which has its own specific rulings. If one prays with the handkerchief without being certain of its impurity, then their prayer is valid. But if one is certain of its impurity and prayed with it forgetfully, it is better to repeat the prayer to avoid scholarly difference of opinion.