Exodus 24:4-6

And Moses wrote down all the words of the LORD. He rose early in the morning and built an altar at the foot of the mountain, and twelve pillars, according to the twelve tribes of Israel. And he sent young men of the people of Israel, who offered burnt offerings and sacrificed peace offerings of oxen to the LORD. And Moses took half of the blood and put it in basins, and half of the blood he threw against the altar.