Riyad as-Salihin 17:62

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] .