Rabban Gamaliel, the son of Rabbi Jehudah the Prince, said: When our father Abraham was circumcised, on the third day he was very sore, in order to test him. What did the Holy One, blessed be He, do? He pierced one hole in the midst of Gehinnom, and He made the day hot, like the day of the wicked. He went forth, and sat down at the entrance of the tent in the cool of the day, as it is said, "And he sat at the tent door (in the heat of the day)" (Gen. 18:1).
The Holy One, blessed be He, said to the ministering angels: Come ye, let us descend and visit the sick, for the virtue of visiting the sick is great before Me. The Holy One, blessed be He, and the angels descended to visit our father Abraham, as it is said, "And the Lord appeared unto him" (ibid.). The Holy One, blessed be He, said to the ministering angels: Come ye and see ye the power of circumcision.
Before Abraham was circumcised he fell on his face (before Me), and afterwards I spake with him, as it is said, "And Abraham fell upon his face" (Gen. 17:17). Now that he is circumcised he sits and I stand. Whence do we know that the Holy One, blessed be He, was standing? Because it is said, "And he looked, and, lo, three men stood over against him" (Gen. 18:2).