Midrasz do Diwrej ha-jamim II 20:5

(Fol. 5b) Now let us see, whence do both R. Juda and R. Simon deduce that one tribe is also called Kahal (congregation)? From the following passage (II Chron. 20, 5) And Jehoshaphat stood forward in the Kahal of Judah and Jerusalem. R. Acha b. Jacob raised the following objection: "Perhaps there it was Kahal, because the tribe Benjamin, too, was there, hence more than one tribe? Therefore," said R. Acha b. Jacob, "it is inferred from the following (Gen. 48, 4) And I will make of thee a Kahal of people; now, at that time, Benjamin had not yet been born, and the Almighty said to Jacob: One Kahal more is born to you [referring to Benjamin]." Said R. Sheba to R. Cahana: "Perhaps the Holy One, praised be He! meant, now as Benjamin is born, and you have twelve tribes you are called Kahal?" Whereupon R. Cahana replied: "Do you mean to say that only twelve tribes are called Kahal, but eleven tribes are not called Kahal?