It's in today's Gospel. Not exactly a "commandment" as in "Thou Shalt Not," this is one of Jesus' teachings on how his disciples ought to live in community. "If your brother sins against you, go and tell him his fault between you and him alone."
The superior of my community in Chicago is especially gifted in this kind of social reconciliation. A few years ago, I came home from a class feeling quite "set upon" by the professor, who tended to respond with sarcasm to any attempt of mine to participate in the program. (I didn't know it at the time, but the prof had had a few run-ins with the Holy See, and as I was the only person in a habit in this class on religious life, I must have been a kind of unwelcome reminder of that experience.) My superior's response to me when I detailed the treatment I had received was, "Why don't you invite her to supper with us?" (I confess that I didn't find it in me to follow this advice.)
Is it just me, or is there something kind of intimidating in Jesus' stipulation "between you and him alone"? When someone has violated my rights or hurt me in some way, my instinctive reaction is to get people "on my side." I want reassurance that, yes, I am the innocent party here; yes, I was set upon in some way; yes, the "other person" bears full responsibility. If I'm going to bring the matter up with the offending party, I want to have supporters backing me up.
If it's just "between you and him alone," we're on equal footing, and the other party may have something valid to say to me. I might learn that I am more than the recipient of an injustice; that I have my own part to play in this little incident; that I have my own apology to make. (Or not, as was the case with the professor and her "issues.")
At any rate, today's Gospel represents a big challenge for me. What about you?