Does a very small spot of stool after istinja' nullify purity, what is the ruling on doubting its source, and what must be done before prayer in this case?

One must beware of intrusive thoughts (waswasah), for they cause anxiety and doubt and make worship burdensome. It is not necessary to search for impurity nor to wipe with a tissue before every prayer unless one is certain that something has exited. If you are certain that urine has exited after performing ablution (wudu'), then it is obligatory to wash whatever the urine has touched and to repeat the ablution. However, if it is merely a doubt, then pay no attention to it. If the exit of feces is confirmed, even if a small amount, then it is obligatory to perform istinja' (cleansing) and the ablution becomes invalid.