(Exodus, Ibid. 18) "And the people stood from afar": twelve mil, the distance of the Israelite encampment. And whence is it derived that this was the distance? From (Numbers 32:49) "And they encamped by the Jordan from Beth Hayeshimoth until Aveil Hashittim in the plains of Moav" (a distance of twelve mil.)
After the Overwhelming Experience of Hearing God's Voice at Sinai
Mekhilta Tractate Bachodesh 9:15