Mekhilta d'Rabbi Yishmael
(Ibid.) "And chamushim did the children of Israel go up from the land of Egypt": "chamushim" indicates "armed," as in (Joshua 1:14) "Then you shall cross over chamushim" (in context, "armed"), and (Ibid. 4:12) "And the children of Reuven and the children of Gad and half the tribe of Menasheh crossed over chamushim … (13) forty thousand armed men, etc."