It has been reported that a man was brought before Abdullah bin Mas'ud (May Allah be pleased with him) because his beard was giving out smell of wine. Ibn Mas'ud said:
"We have been prohibited from spying (on Muslims) and finding faults (with them). But we can take to task only and only if the sin is overt.

[Abu Dawud].

وعن ابن مسعود رضي الله عنه أنه أتي برجل فقيل له، هذا فلان تقطر لحيته خمرًا، فقال: إنَّا قد نهينا عن التجسس، ولكن إن يظهر لنا شئ، نأخذ به". حديث حسن صحيح رواه أبو داود بإسناد على شرط البخاري ومسلم.