What is the ruling on the prayer of one from whom a drop of water falls onto his underwear, and is it permissible to wipe it with water or change clothes, and what if he prayed in this state and then remembered?

If a Muslim is certain that a drop of urine has descended after istinja' (washing after urinating/defecating) and wudu' (ablution), then his wudu' is invalidated, and he must perform istinja' and wudu' again before prayer. If the drop reaches the garment or body, the affected area must be washed, and prayer is not valid before doing so.

It is necessary to wait until urination has completely ceased. As for making the tashahhud or wiping the affected area with water, this is not sufficient; rather, it is an innovation. What is obligatory is to wash the affected area. It is not necessary to change all clothes; washing the contaminated spot is sufficient.

As for one who prayed with impurity out of forgetfulness, his prayer is valid according to many scholars, based on the Almighty's saying: "Our Lord, do not hold us accountable if we forget or make a mistake."