Mishnah Chagigah

One may not expound the topic of forbidden sexual relations before three or more individuals; nor may one expound the act of Creation and the secrets of the beginning of the world before two or more individuals; nor may one expound by oneself the Design of the Divine Chariot, a mystical teaching with regard to the ways God conducts the world, unless he is wise and understands most matters on his own.⁠a The mishna continues in the same vein: Whoever looks at four matters, it would have been better for him had he never entered the world: Anyone who reflects upon what is above the firmament and what is below the earth, what was before Creation, and what will be after the end of the world. And anyone who has no concern for the honor of his Maker, who inquires into and deals with matters not permitted to him, deserves to have never come to the world.