M. Tullius Cicero Cum Populo Gratias Egit 23

[23] Hatred may either be appeased by entreaties, or may be laid aside out of consideration for the emergencies of the republic and the general advantage, or it may be restrained by the difficulty of avenging oneself, or it may be worn out by the antiquity of the injury which gave rise to it; but a man ought not to require to be entreated to show attention to virtuous men, ¹ Nor is the excuse of difficulty to be admitted; nor is it just to limit the recollection of a kindness to a certain time or to a fixed day. Lastly, he who is somewhat indifferent about seeking revenge is soon openly praised; but he is most exceedingly blamed who is in the least slow in requiting such benefits as you have showered on me; and he must inevitably be called, not only ungrateful, which itself is serious enough, but impious also. And the principle of requiting a kindness is different from that of repaying money; because he who keeps the money does not pay it, he who has repaid it has not get it; but in the case of gratitude, he who repays it still keeps it and he who keeps it pays it.