Abu Sa’id Al-Khudri related that the Messenger of Allah (SAW) said: Indeed Satan may come to any one of you as he is praying, and will grab a hair from his rear and stretch it out, and he will think that he has had an occurrence, but he should not leave unless he hears a sound or perceives a smell.
- Musnad Ahmad, Section: The hadith of Abu Sa’id Al-Khudri – May Allah most high be pleased with him

"to see if" : that is, the person praying thinks "he has had an occurrence" from the passing of gas from his rear; and if this occurs "he should not leave" his prayer, that is, he should not abandon it to purify himself and resume again, "unless he hears a sound", that is, the sound of gas exiting him, "or perceives a smell", that is, he smells an odour coming from him. This is a figure of speech expressing the certainty of having had an occurrence, since this is how one becomes aware of it.