מדרש על שמואל א 4:4

(Numb. 31:6, cont.:) AND THE VESSELS OF THE SANCTUARY (haqodesh). This refers to the Ark (and its contents), since it is stated (in Numb. 7:9): BECAUSE THEY (i.e., the children of Korah) HAD THE SERVICE OF THE HOLY (haqodesh) OBJECTS, WHICH THEY CARRIED ON THE SHOULDER.¹⁶The interpretation assumes an analogy with I Sam. 4:4. R. Johanan said: These were the priestly garments, for in them were the Urim and Thummim, as stated (in Exod. 29:29): AND THE SACRED (haqodesh) GARMENTS OF AARON. (Numb. 31:8:) THEY ALSO KILLED BALAAM BEN PEOR WITH THE SWORD. Now what was he doing there? He had simply gone to receive a reward for twenty-four thousand that had fallen through his counsel.