"the oath of the L–rd (Yod-Keh) shall be between the two of them": by Yod-Keh. From here you derive the same for all "oaths" in the Torah. Since all the "oaths in the Torah are unqualified (as to the formula for the oath), and the Torah qualified an oath in one instance as being "Yod-Keh," so all the oaths in the Torah must be by "Yod-Keh."