Sforno on Exodus 24:1/ספורנו על שמות כ״ד:א׳

1ואל משה אמר עלה!, after He had finished telling him כה תאמר אל בני ישראל אתם ראיתם in 20,18. At that time He had explained to the people that they must not try and “reach” Him through intermediaries, but that all that was required was an altar made of earth, coupled with the meticulous observance of His commandments as spelled out in the Ten Commandments. The verse tells us that all of these things were explained to the whole nation. However, ואל משה אמר, to Moses himself G’d had said that he should come up to the mountain just as he had been commanded to do already before the revelation, when G’d had said to him in 19,14 לך רד ועלית אתה ואהרן עמך.