Exodus 17:9

בחר לנו CHOOSE OUT MEN FOR US — for me and for you: he (Moses) placed him (Joshua) on the same level with himself. They (the Rabbis) derived from here the teaching: let thy disciple’s honour be as dear to thee as thy own. And whence may we derive the next statement in thee well-known passage: and thy colleague’s honour as dear to thee as the reverence due to thy teacher? Because it is said, (Numbers 12:11) “And Aaron spake to Moses, “I beseech thee, my lord” — but surely Aaron was older than his brother and yet by addressing him as “my lord” he treated his colleague (Moses) as his teacher! “And the reverence due to thy teacher as the reverence due to God” — whence may this be derived? Because it is said, (Numbers 11:28) “[And he (Joshua) said (to his teacher, Moses)] “My lord, Moses, כלאם”, destroy them (כַּלֵּם) from out of the world! they who rebel against thee deserve to be destroyed just the same as though they had rebelled against the Holy One, blessed be He (Mekhilta). וצא הלחם GO OUT, FIGHT — go out from the cloud and fight against him (Mekhilta), מחר TO-MORROW, at the time of the battle, I will take my stand. בחר לנו אנשים CHOOSE US MEN — brave men, and men who fear sin (אנשים does not mean merely men — males, but men fit for war and at the same time God-fearing) — choose these so that their merit may stand them in good stead (cf. (Mekhilta). Another explanation: בחר לנו אנשים Choose us men who know how to make witchcraft of none effect; he said this because the sons of Amalek were wizards (cf. Rashi on v. 12)

וַיֹּ֨אמֶר מֹשֶׁ֤ה אֶל־יְהוֹשֻׁ֙עַ֙ בְּחַר־לָ֣נוּ אֲנָשִׁ֔ים וְצֵ֖א הִלָּחֵ֣ם בַּעֲמָלֵ֑ק מָחָ֗ר אָנֹכִ֤י נִצָּב֙ עַל־רֹ֣אשׁ הַגִּבְעָ֔ה וּמַטֵּ֥ה הָאֱלֹהִ֖ים בְּיָדִֽי׃

And Moshe said to Yehoshua, Choose us out men, and go out, fight with ῾Amaleq: tomorrow I will stand on the top of the hill with the rod of God in my hand.