Avot DeRabbi Natan

Moses was sanctified by the cloud and received the Torah at Sinai, as it is stated, And the glory of the Lord abode upon mount Sinai, and the cloud covered him six days. [It covered Moses] in order to cleanse him, and this occurred after [the proclamation of] the Ten Commandments. So expounded R. Jose the Galilean. R. Aḳiba said: The verse and the cloud covered him six days refers to [the mountain and not to] Moses. The continuation of the verse, And the seventh day He called unto Moses out of the midst of the cloud, is to single Moses out for special honour.

R. Nathan said: Why was it that Moses remained on the mountain six whole days without the Word descending upon him? It was that he might be purged of all food and drink which were in his bowels, until such time as he would become holy and be like the ministering angels. R. Mathia b. Ḥeresh said to him, ‘Master, surely the only purpose was to impress him so that he should receive the words of the Torah with awe and fear, with dread and trembling, as it is stated, Serve the Lord with fear and rejoice with trembling.