Riyad as-Salihin
Riyad as-Salihin 1572
Prohibition of Spying on Muslims and to be Inquisitive about Others
Arabic hadith text
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.
Translation
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].