Do Not Argue

And all the brothers are to guard against speaking falsely of anyone and are to avoid verbal disputes. Rather, let them seek to keep quiet whenever God gives them the grace to do so. And they are not to argue among themselves, nor with others, but they are to strive to respond with humility, saying, "We are useless servants" (Luke 17:10).

Saint Francis of Assisi
Rule of 1221
Chapter 11