What was kept inside the Ark of the Covenant?

Answer: The stone tablets

God told Moses to put the Testimony in the ark, and Moses placed the stone tablets in it, as the LORD commanded.

“And place inside the ark the Testimony, which I will give you. and I went back down the mountain and placed the tablets in the ark I had made, as the LORD had commanded me; and there they have remained.”— Exodus 25:16; Deuteronomy 10:5 (BSB)