Ambalatthika-rahulovada Sutta: Instructions to Rahula at Mango Stone (MN 61)

"Whenever you want to do a verbal action, you should reflect on it: 'This verbal action I want to do — would it lead to self-affliction, to the affliction of others, or to both? Would it be an unskillful verbal action, with painful consequences, painful results?' If, on reflection, you know that it would lead to self-affliction, to the affliction of others, or to both; it would be an unskillful verbal action with painful consequences, painful results, then any verbal action of that sort is absolutely unfit for you to do. But if on reflection you know that it would not cause affliction... it would be a skillful verbal action with pleasant consequences, pleasant results, then any verbal action of that sort is fit for you to do."