“Monks, these two people are hard to find in the world. Which two? The one who is first to do a kindness, and the one who is grateful for a kindness done and feels obligated to repay it. These two people are hard to find in the world.”
AN2.118: Dullabhā Sutta
Dullabhā Sutta: Hard to Find - translated by Thanissaro Bhikkhu
Translated by Thanissaro Bhikkhu. Source text via dhammatalks.org.