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)
No foul language should come out of your mouths, but only such as is good for needed edification, that it may impart grace to those who hear.
And yet, neither St. James nor St. Paul can seem to sway us. Even on Sunday, some people can't even make it out the doors of the church before they start. Imagine a parish where no one grumbled about anyone else.
Whatever it is, the scriptures are clear, it is a form of darkness that we need to fight on a daily basis. And in some ways it may be the simplest way of avoiding judgment in the end.