This proverb, parable, saying or whatever you want to call it is a very popular one. If you say this to an Arabic speaking friend they will be very impressed with you, even if it's the only thing you know how to say in Arabic!
If you see the fangs of a lion showing, don't think the lion is smiling.
اذا رايت نيوب الليث بارزةً فلا تظن ان الليث يبتسم
There are 2 words used here that have common synonyms. The word you probably know for lion is أسد (asad). ليث means lion as well. Also, the word used here for fangs is نيوب , but أنياب (anyaab) is used as well.