Musannaf Ibn Abi Shayba 3:5625

Hatim ibn Isma'il narrated to us, from 'Abd al-Rahman ibn Harmala, from Sa'id ibn al-Musayyib, that he heard him say: "Indeed, the devil flows within a human being like the flow of blood, then he pulses at his posterior, so he makes him exit (from the state of purity). Therefore, let none of you exit (from prayer) until he hears a sound or finds a smell."