Where does "the unification of the four seas make the world stable" come from? What are the classic sentences in ancient times that discuss the way of governing the country?

This should be from what Mencius said when discussing Wang Jun's way.

Kings are ordered by heaven, govern the world and serve the people, which is really the world. Serving the world is nothing more than unifying the four seas first, so that the world will be stable and the people will have a bumper harvest. After the harvest, food is the priority for the people, and security is the priority for the people. There will be no chaos. On the one hand, the world will be peaceful, and on the other hand, it will benefit the world. Guan Xinmin's people's livelihood, economic development and disaster management are the basis of governing the world.