Exodus 24:4-6

Then Moses wrote it all down, everything GOD had said. He got up early the next morning and built an Altar at the foot of the mountain using twelve pillar-stones for the twelve tribes of Israel. Then he directed young Israelite men to offer Whole-Burnt-Offerings and sacrifice Peace-Offerings of bulls. Moses took half the blood and put it in bowls; the other half he threw against the Altar.