Musannaf Ibn Abi Shayba 17:1266

Abu Mu'awiyah narrated to us, from al-A'mash, from Zayd ibn Wahb, who said: Ibn Mas'ud was approached, and it was said to him, "This is so-and-so, his beard is dripping with wine." Abdullah [ibn Mas'ud] said, "We have been forbidden from spying, but if something becomes apparent to us from him, we will take him to task for it."