Musannaf Ibn Abi Shayba 3:5100

Abu Bakr narrated to us, he said: Abdullah bin Numayr narrated to us, from al-Awza’i, from Hassan bin Atiyyah, he said: Mu’adh bin Jabal used to take a flea while in prayer, and he would rub it with his hand until he killed it, then he would spit upon it.