What should one do if they find traces of urine or faeces on their underwear hours after performing Istinja (ritual cleansing after defecation or urination), and does that nullify Wudu (ablution) or necessitate changing clothes?

If there is a trace of urine or feces on the garment, only the contaminated area needs to be washed.

If the impurity exited before or during the prayer, the prayer must be repeated, as purity is a condition for its validity. However, if it exited after the prayer, it does not affect the validity of the past prayer, and one must wash the impurity and perform ablution for the next prayer.

If there is doubt about the time the impurity exited, the event is attributed to the nearest time, and it is presumed to have exited after the prayer.

If the impurity exits repeatedly, making it impossible to perform prayer with proper purity, the ruling for someone with incontinence (Sahib al-Salas) applies: one should secure a cloth, perform ablution for each prayer after its time has begun, and pray both obligatory and supererogatory prayers with this ablution.