Exodus 24:3-5

Young's Literal Translation

³ And Moses cometh in, and recounteth to the people all the words of Jehovah, and all the judgments, and all the people answer -- one voice, and say, `All the words which Jehovah hath spoken we do.'

⁴ And Moses writeth all the words of Jehovah, and riseth early in the morning, and buildeth an altar under the hill, and twelve standing pillars for the twelve tribes of Israel;

⁵ and he sendeth the youths of the sons of Israel, and they cause burnt-offerings to ascend, and sacrifice sacrifices of peace-offerings to Jehovah -- calves.