Musannaf Ibn Abi Shayba 1:1081

Ghundar narrated to us, from Shu'bah, from Hammad, from Ibrahim, that Ibn Mas'ud was walking towards the Euphrates while he was teaching a man to recite. Ibn Mas'ud urinated, so the man refrained from him. Ibn Mas'ud said, "What is the matter with you?" He said, "You have urinated." Ibn Mas'ud said, "I am not in a state of major ritual impurity (junub)."