Bamidbar 31:5

וימסרו SO THERE WERE HANDED OVER … [A THOUSAND OF EVERY TRIBE] — This serves to tell you in what high esteem the shepherds of Israel were really held — how dear they were to Israel. Up until they had heard of his approaching death what does it state? That Moses felt constrained to exclaim about them, “They are almost ready to stone me”! (Exodus 17:14). But when they heard that Moses’ death was associated with the execution of vengeance upon Midian (cf. v. 2), they refused to go to war until they were “handed over” against their own will (Sifrei Bamidbar 157:3).

So there were delivered out of the thousands of Yisra᾽el, a thousand of every tribe, twelve thousand armed for war.

וַיִּמָּֽסְרוּ֙ מֵאַלְפֵ֣י יִשְׂרָאֵ֔ל אֶ֖לֶף לַמַּטֶּ֑ה שְׁנֵים־עָשָׂ֥ר אֶ֖לֶף חֲלוּצֵ֥י צָבָֽא׃