References – AN 7.52: A Very Fruitful Gift 
“When was the idea of merit [being like a celestial currency] incorporated into Buddhism?” Answered by Ajahn Amaro, Ajahn Karuṇadhammo and Ajahn Sundarā. [Merit] [History/Early Buddhism] // [Generosity] [Happiness]
Sutta: Iti 22: “Don’t belittle merit.”
Sutta: AN 7.52: Degrees of meritorious offerings.