Mekhilta de-Rabbi Shim‘on ben Yohai, ed. J. N. Epstein and E. Z. Melamed (Jerusalem, 1955), p. 81.

Jewish Mysticism
Another interpretation- “YHWH is a man of war, YHWH is His name.” Because when the Holy One, blessed be He, was revealed at the sea He appeared as a young man making war. “YHWH is His name-” He appeared to them at Sinai like an old man full of mercy. “As I looked on, thrones were set in place” (Dan. 7-9). So as not to give an opportunity [for one] to say, “There are two powers in heaven.” Rather “YHWH is a man of war.”

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.