Sahih Bukhari
Sales and Trade
Volume: 003 / Book Number: 034 / Hadith Number: 314
Narrated Abu Musa:
Allah's Apostle said, "The example of a good companion (who sits with you) in comparison with a bad one, is I like that of the musk seller and the blacksmith's bellows (or furnace); from the first you would either buy musk or enjoy its good smell while the bellows would either burn your clothes or your house, or you get a bad nasty smell thereof."