138. "And he said, 'I will certainly return to you at this season...'" (Beresheet 18:10). Rabbi Yitzchak asks, Why is it written, "I will certainly return?" I should have said, 'He will certainly return,' as the key to impregnating barren women is in the hands of the Holy One, blessed be He, and not in the hands of any other messenger.
139. As we have learned, there are three keys that were not handed over to any messenger, the keys of life, of the resurrection of the dead, and of the rains. As they were not handed over to any messenger, why is it written "I will certainly return," WHICH MEANS THAT THE ANGEL WILL RETURN "AT THIS TIME" AND VISIT HER? AND HE REPLIES, It is clear that the Holy One, blessed be He, who stood by them said this phrase. This is why it is written, "I will certainly return to you."
140. Come and behold, wherever it is merely written, "And he said" or "And he called," WITHOUT MENTIONING WHO SAID OR CALLED, it is a reference to the Angel of the Covenant, NAMELY THE SHECHINAH, and no other. "And he said..." appears in the verse "And he said, if you will diligently hearken to the voice..." (Shemot 15:26) but the verse does not mention who said this. It is also written in the verse "And he called upon Moses..." (Vayikra 1:1) but again, it does not say who called. Again, it is written, "And to Moses, he said..." (Shemot 24:1) but it does not say who. In all these places, it is the Angel of the Covenant, NAMELY THE SHECHINAH. And everything has been said in reference to the Holy One, blessed be He, BECAUSE THE SHECHINAH IS THE HOLY ONE, BLESSED BE HE. This is why it is written, "And he said, I will certainly return to you...and, lo, Sarah your wife shall have a son." THUS, THE HOLY ONE, BLESSED BE HE, WHO HAS THE KEY FOR IMPREGNATING BARREN WOMEN IN HIS HANDS, ALONE MAY SAY, "I WILL CERTAINLY RETURN..."