Is the prayer considered valid if one performed istinja' (ablution) initially and offered the prayer, then found impurity after the prayer?

It appears that the questioner, who suffers from waswasah (obsessive whispering), finds wetness after relieving himself. The remedy for this is to sprinkle water on the private parts or clothing after istibra' (ensuring no urine drops remain) to cut off the waswasah. These narrations indicate that a person is not required to follow delusions and waswasah. If wetness is found and one is not certain that it is an impurity, it should be disregarded, and the prayer is valid. However, if the presence of urine or faecal impurity is confirmed after istinja' (cleansing after defecation/urination) and before prayer, then it must be washed, and ablution performed.