No matter who is initiating the breakup, keep a civil tongue. Poisonous words cast during the breakup process may come back to haunt you. (Google)
"Come in then and when you ask favors learn to keep a civil tongue in your head," said the woman sternly, taking the guest by the hand and pulling her in. (Googlebooks)
Please keep a civil tongue in your mouth and treat others with respect even if you disagree with them. (Google)