Midrasch zu Dewarim 3:18

(Bamidbar 31:2) "Afterwards you will be gathered unto your people": We are hereby apprised that Moses' death was contingent upon the defeat of Midian — in spite of which he entered into it with zeal, viz. (Ibid. 3) "And Moses spoke to the people, saying 'hechaltzu,'" connoting zeal, as in (Devarim 3:18) "Chalutzim shall you cross over." (Ibid.) "And let them be against Midian to execute the vengeance of the L-rd against Midian": He told them: You are not executing the vengeance of flesh and blood, but the vengeance of Him who spoke and brought the world into being, as it is written (Nachum 1:2) "the L-rd is a G-d of scorn and revenge."