הלכה על במדבר 1:47

On the other hand, whenever the Torah takes a census of those who are fit to wage war ("kol yotzei tzava"), it excludes the tribe of Levi, implying that this tribe does not fight in the army. The Rashbam (Bemidbar 1:47) even refers explicitly to their exclusion for the army. Hence, using the Levites as a paradigm for excusing Torah scholars from serving in the army remains debatable, for the status of Levi is itself uncertain.⁴For a complete discussion of the status of Levi in the army, see Rav Yehoshua Hagar-Lau's Ha'oz Veha'anavah (pp.127-141).