Exodus 20:18

נגש אל הערפל [AND MOSES] STEPPED NEAR TO THE THICK CLOUD — within the three divisions — darkness, cloud and thick cloud (Mekhilta), as it is said, (Deuteronomy 4:11) “And the mountain burned with fire unto the midst of the heaven, with darkness, clouds and dark clouds”. The ערפל is the עב הענן of which God spoke to him when He said, (19:9) “Behold I come unto thee in a thick cloud (עב הענן)” (cf. Rashi on that verse).

וַיַּעֲמֹ֥ד הָעָ֖ם מֵרָחֹ֑ק וּמֹשֶׁה֙ נִגַּ֣שׁ אֶל־הָֽעֲרָפֶ֔ל אֲשֶׁר־שָׁ֖ם הָאֱלֹהִֽים׃ {ס}        

And the people stood afar off, and Moshe drew near to the thick darkness where God was.