וַיֵּ֥רֶד מֹשֶׁ֖ה אֶל־הָעָ֑ם וַיֹּ֖אמֶר אֲלֵהֶֽם׃ (ס)
So Moses went down unto the people, and told them.
Sforno on Exodus
ויאמר אלהם, he conveyed G’d’s warning to them as well as the penalty for violating G’d’s command. (verse 21)
Rashbam on Exodus
ויאמר אליהם, he told them of the commandment to fence off the mountain as of immediately.