What is the ruling on traces of impurity remaining after cleaning oneself with water (istinja') following defecation, knowing that it is a light color that appears on a white tissue and requires time to remove with water, and is prayer valid with it?

The sufficient limit for istinja' (cleansing after defecation or urination) with water is the removal of the substance of the impurity and its traces, and the return of the area to its original rough state. As for the limit for istinja' with stones, it is that nothing remains but a trace that can only be removed with water. The color of the impurity must be removed if that is easy, because its persistence indicates the persistence of the substance of the impurity. A trace of impurity is only excused in the case of istijmar (cleansing with stones or similar dry materials); the remaining trace after thorough cleansing and exhaustive effort is excused, and prayer is valid with it. However, if the trace is not excused, then prayer is not valid according to most scholars.