Vayikra Rabbah 33:1 (Behar)
Rabban Gamaliel said to Tavi his servant: ' Go and get me good food from the market.’ He went and bought him tongue. He said to him: ‘Go and get me bad food from the market.’ He went and bought him tongue. Said he to him: ‘What is this? When I told you to get good food you bought me tongue, and when I told you to get bad food you also bought me tongue! ' He replied: ' Good comes from it and bad comes from it. When the tongue is good there is nothing better, and when it is bad there is nothing worse.’
Rabi made a feast for his disciples and placed before them tender tongues and hard tongues. They began selecting the tender ones, leaving the hard ones alone. Said he to them: Note what you are doing! As you select the tender and leave the hard, so let your tongues be tender to one another
Accordingly Moshe admonishes Israel by saying: AND IF THOU SELL AUGHT... YE SHALL NOT WRONG ONE ANOTHER. (i.e. Moshe encourages the Israelites to be careful with what they say to each other)
(Translation, based on Judaica classics)
