For there is no more essential duty than that of returning kindness received.

[Nullum enim officium referenda gratia magis necessarium est.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
De Officiis [On Duties; On Moral Duty; The Offices], Book 1, ch. 15 / sec. 47 (44 BC) [tr. Peabody (1883)]
    (Source)

(Source (Latin)). Alternate translations:

For of all the virtues, there is none we are more necessarily obliged to, than gratitude.
[tr. Cockman (1699)]

For there is no duty of a more necessary obligation than returning a kindness.
[tr. McCartney (1798)]

For there is no duty more indispensable than that of returning a kindness.
[tr. Edmonds (1865)]

For no duty is more imperative than gratitude.
[tr. Gardiner (1899)]

There is no duty more obligatory than the repayment of a kindness.
[ed. Harbottle (1906)]

For no duty is more imperative than that of proving one's gratitude.
[tr. Miller (1913)]

No duty is more necessary than to return a favor.
[tr. Edinger (1974)]