Rashi on Genesis 17:3/רש"י על בראשית י״ז:ג׳

1ויפל 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).