Reference for Chagigah 2:1
אֵין דּוֹרְשִׁין בַּעֲרָיוֹת בִּשְׁלֹשָׁה. וְלֹא בְמַעֲשֵׂה בְרֵאשִׁית בִּשְׁנַיִם. וְלֹא בַמֶּרְכָּבָה בְּיָחִיד, אֶלָּא אִם כֵּן הָיָה חָכָם וּמֵבִין מִדַּעְתּוֹ. כָּל הַמִּסְתַּכֵּל בְּאַרְבָּעָה דְּבָרִים, רָאוּי לוֹ כְּאִלּוּ לֹא בָּא לָעוֹלָם, מַה לְּמַעְלָה, מַה לְּמַטָּה, מַה לְּפָנִים, וּמַה לְּאָחוֹר. וְכָל שֶׁלֹּא חָס עַל כְּבוֹד קוֹנוֹ, רָאוּי לוֹ שֶׁלֹּא בָּא לָעוֹלָם:
Illicit relations are not expounded to three [men at the same time. Recondite matters of Torah are not expounded to them, e.g., the interdict against living with one's daughter from a woman he had ravished, which is not explicitly stated in Scripture, but is derived exegetically. For while the teacher is addressing one of them the other two might be conversing and not give heart to the teacher's expounding of the interdict, so that they might come to be remiss in the area of illicit relations. For these things are more lusted after and desired than the other things forbidden by the Torah.] And the [six days of] creation (are not expounded) [even] to two [and, it goes without saying, to three or more, it being written (Deuteronomy 4:32): "For ask (singular), now, of the first days" (of creation) — One may inquire, but not two.] And the episode of the Divine Chariot [beheld by Ezekiel and by Isaiah] (may not be expounded) [even] to one, unless he be a sage, "understanding by himself" [i.e., unless the teacher recognizes that he is a sage, who, if given the "outlines" of things, will understand the rest by himself. [Rambam explains "the creation" as natural wisdom, and "the episode of the Divine Chariot" as the existence of G d, His attributes, the angels, the soul, the intellect, and what transpires after death. It does not seem to me that all of this is subsumed in the "episode of the Divine Chariot." The "wisdom of the Divine Chariot" would be more apt. It is called, rather, the "episode of the Divine Chariot" because by invoking certain holy names one has recourse to the Crown, by whose agency he beholds the angelic watches in their stations and "sanctuary within sanctuary," as those who see (such things) through the agency of the holy spirit.] All who reflect upon four things [those that follow] — it would have been better had they not been born: what is above [the heads of the heavenly creatures], what is below, what is before, and what is behind [to the west. Another interpretation: "what is before" the creation; "what is behind" the creation, at the end of days.] And all who are not solicitous of the honor of their Creator [such as those who transgress in secret, saying: "The Shechinah is not found here. Who sees me? Who knows me?"] — it would have been better had they not been born.