Shemot 24:17-18

And the sight of the Kevod HASHEM was like eish ochelet (devouring fire) on the top of HaHar in the eyes of the Bnei Yisroel. And Moshe went into the midst of the anan, going up into HaHar; and Moshe was in HaHar arba'im yom v'arba'im lailah.