And Rabbi Ḥiyya bar Abba also said that Rabbi Yoḥanan said: Had there been left open a crack so much as the size of small sewing needle in the cave in which Moses and Elijah stood when G-d’s glory was revealed to them, as it is written: “And it shall come to pass, while My glory passes by, that I will put you in a cleft of the rock” (Exodus 33:22), and: “And he came there to a cave…and, behold, the L-rd passed by” (I Kings 19:9–11), they would not have been able to endure due to the intense light that would have entered that crack, as it is stated: “For no man shall see Me and live” (Exodus 33:20).
– Megillah 19b; sefaria.org
The Tree of Life 