I'm sure this has been quoted a million times all over the internet, but this is the first time I've read it. This is the first paragraph in Jonathan Edwards' "Discourse on the Trinity":
When we speak of God's happiness, the account that we are wont to give of it is that God is infinitely happy in the enjoyment of himself, in perfectly beholding and infinitely loving, and rejoicing in, his own essence and perfections. And accordingly it must be supposed that God perpetually and eternally has a most perfect idea of himself, as it were an exact image and representation of himself ever before him and in actual view. And from hence arises a most pure and perfect energy in the God-head, which is the divine love, complacence and joy.
Edwards, Jonathan The Works of Jonathan Edwards Vol. 21: Writings on the Trinity, Grace, and Faith ed. Sang Hyun Lee (New Haven: Yale University Press, 2003), 113.