AN 119PTS: A i 87
Dullabha Sutta
Hard to Find
Translated from the Pāli by Thanissaro Bhikkhu
"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."