Ibn Masud was approached and told, "This is so-and-so, with wine dripping from his beard."Abdullah replied, "We have been forbidden from spying, but if something becomes openly apparent to us, we will act upon it."
حدثنا أبو بكر بن أبي شيبة حدثنا أبو معاوية عن الأعمش
أتي ابن مسعود فقيل هذا فلان تقطر لحيته خمرا فقال عبد الله إنا قد نهينا عن التجسس ولكن إن يظهر لنا شيء نأخذ به
[Its chain of transmission is daif] Abdul-Razzaq narrated it, no. 18945, from the hadith of Amash, and Ibn Abdul-Barr in Tamhid, 21/18, 22, from the hadith of Abu Dawud via him. Amash was a mudallis and narrated using "an" (indicating possible concealment in transmission).