Rashi on Yisro 19:12

English: והגבלת AND THOU SHALT SET BOUNDS — set boundary marks for them as a sign that they should not approach the mountain from the boundary and beyond. לאמר SAYING — The boundary says to them, “Take care not to ascend from here and further on” and you, too, admonish them regarding this. ונגע בקצהו OR TOUCH THE EXTREMITY THEREOF — i. e. even the extremity thereof.