Today's first reading opens with just such a command from St. James
Do not grumble against one another, brothers and sisters, so that you may not be judged.
The Greek has that sense of muttering under your breath. All to often we don't reserve the comment to under our breath; we say it out loud, which is even worse.
Not only does St. James tells what not to do, but he tells us the reward we get for not doing it. Don't grumble against others, and you can avoid judgement. And yet, even after reading thi,s how many of will make to bed this evening without falling into this sin, without having to comment negatively about somebody.
St. Paul gives the Ephesians the same command (Eph. 4:29)