Shemot 24:15-18

And Moshe went up into HaHar, and an anan concealed HaHar. And the Kevod HASHEM abode upon Mt. Sinai, and the anan concealed it sheshet yamim; and the yom hashevi'i He called unto Moshe out of the midst of the anan. 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.