What is the ruling on prayer when a person has some doubt, such as concerning purity?

The state of one who doubts during ablution is one of two:

1. If he is certain of his purity but doubts the occurrence of an impurity: He should disregard his doubt and consider his ablution valid, based on the Prophet, peace and blessings be upon him, who said concerning one who feels something in his stomach and doubts whether anything came out of him: "He should not leave (the prayer) until he hears a sound or perceives a smell." 2. If he is certain of an impurity but doubts the occurrence of purity: His ablution is considered void, and he must repeat it.