שמואל ב יח:כא מדרש: Pirkei DeRabbi Eliezer

One Scripture text says, "Then said Joab to the Cushite, Go, tell the king what thou hast seen" (2 Sam. 18:21). Was he a Cushite? Was he not a Benjamite? But just as this Cushite is different from all creatures, so was the Benjamite || different by his ways and his good deeds; therefore was his name called "Cushite."