Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Satan circles around a man during his prayer in order to break his prayer for him. When he grows weary, he blows into his rear. So when one of you senses that, let him not turn away from the prayer until he perceives a smell or hears a sound."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9199