Numbers 16:4

And when Moses heard [it]
What they said, their complaint against him, for setting up Aaron for an high priest, and against Aaron for taking this honour to himself:

he fell upon his face;
through shame, as the Targum of Jonathan, blushing at their sin, in opposing the ordinance of God; and through fear of the divine displeasure, and of the wrath of God coming upon them for such wickedness; and in order to pray to God for them to make them sensible of their sin, and give them repentance for it, and pardon of it, and avert his judgments from them such a conduct called aloud for.