שמות יט:כה פירוש: רש״י, ספורנו, רשב״ם וחזקוני

ויאמר אליהם AND HE SPOKE UNTO THEM this warning.

ויאמר אלהם, he conveyed G’d’s warning to them as well as the penalty for violating G’d’s command. (verse 21)

ויאמר אליהם, he told them of the commandment to fence off the mountain as of immediately.