Question: What is the ruling concerning a person who doubts whether he nullified his ablution or not?

Answer: If a person has doubt whether he nullified his ablution or not, then his original state of being pure remains and his doubt does not have any effect. This is based on the Prophet's statement, when a man asked him about feelings he has in his stomach during the prayer,

"Do not leave [the prayer] until you hear a sound or you perceive a smell."

The Prophet (peace be upon him) made it clear to him that the basic ruling is that of purity until one is certain that something has occurred to nullify it. As long as the person is simply in doubt, his state of purity is sound and confirmed. Therefore, he may continue to pray, circumambulate the Kaaba, read the Quran from a mushaf and so forth. That is the original ruling. This is, all praises are due to Allah, from the magnanimity and easiness of Islam.

Shaikh Ibn Baz