What is the ruling on the prayer of one who finds traces of stool in his underwear after purification, and is he required to change clothes for prayer every time?

After proper istinja' (cleansing oneself after defecation or urination), it is not obligatory to search for traces of impurity to avoid waswasah (whispers of Satan leading to obsessive doubts). However, if the presence of traces of feces on the underwear is confirmed, it is obligatory to wash the affected area of the body and the garment. The emission of feces nullifies wudu' (ablution). If this recurs, the garment must be washed or changed before prayer. But if this condition is constant for most of the time, and one cannot purify oneself properly within the prayer time, then your ruling is that of a person afflicted with salasil (incontinence). In this case, you should perform wudu' for each prayer after its time has begun, while taking precautions such as tying a cloth or similar measure.