What is the ruling on one who continued to pray without cleansing himself from urine (Istibra') while being unaware?

Istibra' (ensuring purity after urination) is seeking to clear the urinary tract from impurity. This is done by waiting for the urine to completely exit, and one must be careful not to be negligent in this matter, as it causes torment in the grave. However, one must also beware of extremism and excessive scrupulosity, which lead to waswas (obsessive doubts). If a Muslim relieves himself and is reasonably sure that the urine has stopped, and then performs wudu' (ablution), he should not pay attention to doubts unless he is certain that something has exited. If someone prayed while ignorant of the ruling of istibra', his prayer is valid.