On Benefits Book II, XXII

When we have decided to accept, let us accept with cheerfulness, showing pleasure, and letting the giver see it, so that he may at once receive some return for his goodness: for as it is a good reason for rejoicing to see our friend happy, it is a better one to have made him so.

Let us, therefore, show how acceptable a gift is by loudly expressing our gratitude for it; and let us do so, not only in the hearing of the giver, but everywhere.

He who receives a benefit with gratitude, repays the first instalment of it.