I am looking for the source and the exact details of a discourse by the Buddha. I may not be accurate, and I vaguely remember reading some post wherein Buddha explained how difficult and subtle it is to understand the laws of Kamma. The text that I read was in the form of questions and answers, in which a question was like “Will you believe if an elephant pass through the hole of a needle …”. Buddha answers that in comparison to believing all such things, it is more unbelievable if someone claims to understand the laws of Kamma.
The introduction to the laws of kamma is found in context with the complete path in “In the Buddha’s Words”—Bikkhu Bodhi:
V. The Way to a Fortunate Rebirth
The Law of Kamma
(1) Four Kinds of Kamma (AN 4.232) SCATI
(2) Why Beings Fare as They Do after Death (MN 41) SC
(3) Kamma and Its Fruits (MN 135) SC
“Heedfulness, great king. Just as the footprints of all living beings with legs can be encompassed by the footprint of the elephant, and the elephant’s footprint is declared to be supreme among them in terms of its great size; in the same way, heedfulness is the one quality that keeps both kinds of benefits secure — those in the present life & those in the future life.”—SN 3.17