Commentaire sur La Genèse 17:3

ויפל אברם על פניו AND ABRAM FELL UPON HIS FACE through fear of the Shechinah, because before he was circumcised he did not have the strength to stand on his feet whilst the Holy Spirit stood over him; and it is to this that what is stated in the case of the heathen Balaam refers, (Numbers 24:4) “fallen down, yet with open eyes”. This is to be found in Baraitha of R. Eliezer (Pirkei DeRabbi Eliezer 29).

AND ABRAM FELL ON HIS FACE. The purpose of this expression was to direct his mind towards the prophecy. When the prophecy concerning the commandment of circumcision was completed, Abraham rose and stood. When the word from heaven came to him a second time, saying to him, As for Sarai thy wife, etc.,³⁹⁷Further, Verse 15. he once again fell on his face to direct his mind towards the prophecy, and he also prayed concerning Ishmael.³⁹⁸Verse 18 here. in line with the verses: And they fell upon their faces, and said, O G-d, the G-d of the spirits, etc.;³⁹⁹Numbers 16:22. That I may consume them in a moment. And they fell upon their faces.⁴⁰⁰Ibid., Verses 21-22.

ויפול אברם על פניו, this was a voluntary act demonstrating his gratitude to G'd for entering into the covenant with him.