Halakhah su Numeri 31:4

The Talmud never states explicitly that the Levites did not serve in the army. The Sifrei (commenting on Bemidbar 31:4) addresses this issue regarding the war between the Jews and Midyan, but textual variants lead to opposing conclusions. Rashi's text of the Sifrei (in his commentary on that verse), understands that the Torah includes ("lerabot") Levi in the army that fought against Midyan. However, the Gra's text of the Sifrei reads "to exclude (lehotzi) the tribe of Levi" from that war. This passage in the Sifrei thus proves nothing about Levi's role in the army.