What is the ruling on one who doubts the presence of impurity, such as urine, on their garment after waking up?

The fundamental principle regarding things is purity, and nothing is judged to be impure except with a legal (Shar'i) proof. This presumed purity is certain and does not cease merely by doubt, for certainty is not dispelled by doubt. The evidence for this is the Prophet's (peace be upon him) saying to one who complained that he felt something during prayer: "He should not leave (the prayer) until he hears a sound or perceives an odor."