Musannaf Ibn Abi Shayba 1:1738

Waki’ narrated to us, from Sufyan, from Abu Qays, from Hudhayl, that his brother, Arqam ibn Sharhabil, asked Ibn Mas’ud and said: "I scratch, and my hand reaches my private part." Ibn Mas’ud said: "If you know that a piece of you is impure, then cut it off."