A good friend and a bad friend are like a perfume

On the authority of Abu Musa al-Ash'ari رضي الله عنه the Prophet صلى الله عليه وسلم said:

"A good friend and a bad friend are like a perfume-seller and a blacksmith: The perfume-seller might give you some perfume as a gift, or you might buy some from him, or at least you might smell its fragrance. As for the blacksmith, he might singe your clothes, and at the very least you will breathe in the fumes of the furnace."
Bukhari and Muslim.

An-Nawawi, rahimahullah, said “There is a desirable quality in associating with righteous people, (that possess with them) integrity, decency, piety, knowledge, and good manners. Likewise, there is a prohibition of associating with people of wickedness, innovations (in the religion), (likewise) people who backbite others, people (who are known) to commit a shameful amount of sins, and people with plenty of free time.”

This hadith encourages the companionship of the righteous and warns against the companionship of the wicked! The Messenger of Allaah, (salla Allahu alayhe wa sallam), used these two examples (i.e. the carrier of misk and a blacksmith) in order to clarify (for us) the clear distinction between the two.