Does purity relate only to outer garments, or does it also include inner garments? Is it sufficient to perform ablution (wudu') only, or must one refrain from prayer and change clothes when there are traces of feces with a slight odor due to friction and sweating? What is the ruling on previous prayers, and must they be made up?

If you are not certain that what you found on your clothes is urine, feces, or an impurity like blood or pus, then you are not required to wash it, as it is pure. Consequently, it does not invalidate your ablution (wudu'), because the default state of things is purity, and something is not deemed impure except with certain verification.

One must be cautious of obsessive doubts and uncertainties. If you are not certain about the impurity of what is found on your undergarments, then your ablution is not invalidated. You are not required to remove or change those clothes. It is not permissible for you to delay prayer beyond its time based on this illusion, nor are you required to make up previous prayers.

For the prayer to be valid, all clothes worn by theperformer of prayer, whether inner or outer, must be pure.