AN 2.119
PTS: A i 87
II,xi,2
Dullabha Sutta: Hard to Find
translated from the Pali 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."