Albert Barnes Commentary Psalms 105:9

"[The covenant] which he made with Abraham, And his oath unto Isaac," — Psalms 105:9 (ASV)

Which covenant He made with Abraham — which He “ratified” with Abraham. Literally, “which He cut with Abraham” (Genesis 17:2–14). Compare the notes at Psalms 50:5.

And His oath to Isaac — confirming the promise made to Abraham. See Genesis 26:2-5.