Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Satan circles around a man in the prayer in order to break his prayer for him. When he grows weary, he blows into his rear so that it seems to him that his ablution has been broken. When he senses something of that, let him not turn away until he hears a sound or perceives a smell."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9200