What did God ask Abraham to sacrifice?

Answer: His son Isaac

God tested Abraham by asking him to sacrifice his son Isaac.

“Some time later God tested Abraham and said to him, “Abraham!” “Here I am,” he answered. “Take your son,” God said, “your only son Isaac, whom you love, and go to the land of Moriah. Offer him there as a burnt offering on one of the mountains, which I will show you.””— Genesis 22:1-2 (BSB)