"Sarah was listening at the entrance of the tent, which was behind him." (Gen. 18:10)

What is the meaning of "which was behind him?" It should have said, "She was behind him." This indicates a hidden teaching: "And Sarah heard what the Entrance of the Tent was saying."

That is, Sarah heard the announcement of the Shechinah, which is referred to here as "the Entrance of the Tent," that she would have a son.

This is a lower level of the revelation of the Shechinah, merely the beginning of faith, whereas Abraham was privy to a higher level.

Therefore the verse states, "the entrance of the tent…was behind [i.e. below] him.

Now this presents a difficulty, for Sarah actually achieved a greater level of prophecy that Abraham, as we see from the verse, "And G‑d said to Abraham…, 'Everything which Sarah tells you, obey her words'" (Gen. 21:12) upon which Rashi comments: "This teaches us that Abraham was subordinate to Sarah in prophecy," Moreover, being a woman, she did not require circumcision prior to receiving prophecy, as Abraham did. The reason for this is that...

From the day Sarah was born, she never before heard the word of G‑d Himself, and since this was the first time, only a lower level of prophecy was revealed to her. Sarah was the only woman to whom G‑d ever spoke directly.

[From Zohar I, 103a; selected, translated and annotated by Moshe Miller. Published by Fiftieth Gate Publications and Seminars.]