Roman Catechism 23.11

Chapter 23 — Part III: The Decalogue — Introduction on the Commandments in General

Section 11. All were commanded by God that for three days before the promulgation of the Law they should wash their garments and abstain from conjugal intercourse, in order that they might be more holy and better prepared to receive the Law, and that on the third day they should be in readiness When they had reached the mountain from which the Lord was to deliver the Law by Moses, Moses alone was commanded to ascend the mountain. Thither came God with great majesty, filling the place with thunder and lightning, with fire and dense clouds, and began to speak to Moses, and delivered to him the Commandments