Then, the Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, said: "Similarly, if one is certain of having invalidated their ablution but doubts their purification, it is obligatory upon them to renew their ablution to attain certainty in purification. Prayer is not valid when there is doubt regarding purification. This principle should be understood so that actions are based upon it. We have explained that it is obligatory upon a person not to enter prayer unless they are in a state of purity. If one is certain that they have invalidated their ablution, they should not disregard this certainty unless they are certain that they have purified themselves."
Commentary #55