Musannaf Ibn Abi Shayba 1:3

Muhammad ibn Bishr al-‘Abdi narrated to us, from ‘Abd al-‘Aziz ibn ‘Umar, he said: al-Hasan ibn Muslim ibn Yanaq narrated to me, from a man among the companions of ‘Abdullah ibn Mas‘ud, he said: ‘Abdullah said: "When you enter the privy and intend to uncover yourself, say: 'O Allah, I seek refuge in You from the filthy, the impure, the foul, the foul things, and the accursed Satan.'"