Perhaps you will find it in Chapter XVII of Leviathan,
(that is a "clickable link") which has a line to the effect that:
". . . the laws of nature, as justice, equity, modesty, mercy, and, in sum, doing to others as we would be done to, of themselves, without the terror of some power to cause them to be observed, are contrary to our natural passions, that carry us to partiality, pride, revenge, and the like. And covenants, without the sword, are but words and of no strength to secure a man at all."
I cannot say wheher or not that is what you are looking for, but it probably will be found in that chapter.