A man learned that a prayer he had performed was invalid because he had not performed the correct ablution; what is required of him?

The questioner : All right, a man learned that a prayer he had performed was invalid; what is required of him? And what is required of him if he learns that several prayers were missed and he cannot determine their exact number?

The Shaykh : What was the cause of the invalidity?

The questioner : That the ablution was not correct, O Shaykh.

The Shaykh : He must repeat it.

The questioner : If he is unable to determine their exact number.

The Shaykh: Yes.

The questioner : If he is unable to determine their exact number.

The Shaykh : If he is unable to what?

The questioner : If he is unable to determine their exact number.

The Shaykh : He should increase his prayers until he is convinced that he has fulfilled what is required of him.

The questioner : Yes.