Albert Barnes Commentary Psalms 106:31

"And that was reckoned unto him for righteousness, Unto all generations for evermore." — Psalms 106:31 (ASV)

And that was counted unto him for righteousness - See Numbers 25:11-13. Compare the notes at Romans 4:3. The meaning here is that this was regarded as a “proof” or “demonstration” that he was a righteous man - a man fearing God.

Unto all generations for evermore - Hebrew, “To generation and generation forever.” The record would be transmitted from one generation to another, without any intermission, and would be permanent. This is one of the illustrations of the statement so frequently made in the Scriptures (Deuteronomy 7:9; Romans 11:28) that the blessings of religion will descend to a distant posterity. Such instances are constantly occurring, and there is no legacy which a man can leave his family so valuable as the fact that he himself fears God and keeps his laws.