Where did the second set of Ten Commandments come from, after Moses broke the first set?
The Ten Commandments, recorded in Exodus 20, were originally spoken to the people by God, but the people found this too disturbing and asked Moses to intercede between God and them (Exodus 20:18-21). The remainder of the law was given to Moses and he wrote them down for the people. God then told Moses to return to the mountain. "Then the LORD said to Moses, "Come up to Me on the mountain and be there; and I will give you tablets of stone, and the law and commandments which I have written, that you may teach them."" (Exodus 24:12).
At the end of forty days of instruction, God gave Moses tablets with the law. "And when He had made an end of speaking with him on Mount Sinai, He gave Moses two tablets of the Testimony, tablets of stone, written with the finger of God" (Exodus 31:18). It is these tablets that Moses broke when he came back down from the mountain. "And Moses turned and went down from the mountain, and the two tablets of the Testimony were in his hand. The tablets were written on both sides; on the one side and on the other they were written. Now the tablets were the work of God, and the writing was the writing of God engraved on the tablets. And when Joshua heard the noise of the people as they shouted, he said to Moses, "There is a noise of war in the camp." But he said: "It is not the noise of the shout of victory, Nor the noise of the cry of defeat, But the sound of singing I hear." So it was, as soon as he came near the camp, that he saw the calf and the dancing. So Moses' anger became hot, and he cast the tablets out of his hands and broke them at the foot of the mountain" (Exodus 32:15-19).
God then told Moses to cut two more tablets to replace the ones he broke. "And the LORD said to Moses, "Cut two tablets of stone like the first ones, and I will write on these tablets the words that were on the first tablets which you broke. ... So he cut two tablets of stone like the first ones. Then Moses rose early in the morning and went up Mount Sinai, as the LORD had commanded him; and he took in his hand the two tablets of stone" (Exodus 34:1, 4). "So he was there with the LORD forty days and forty nights; he neither ate bread nor drank water. And He wrote on the tablets the words of the covenant, the Ten Commandments" (Exodus 34:28).
This second set was placed in the Ark of the Covenant.