Bereshit 35:23

בכור יעקב JACOB’S FIRSTBORN — even when it speaks of his corrupt action Scripture calls him firstborn (Genesis Rabbah 82:11). בכור יעקב JACOB’S FIRSTBORN — firstborn with regard to heritage, firstborn with regard to Divine Service (for before the Levites were appointed to minister to God the duty devolved upon the oldest son in each family), firstborn in any enumeration of the twelve tribes; for the right of a firstborn son was given to Joseph only in respect of the tribes — in that he founded two tribes (just as a firstborn son received a double portion in his father’s property) (Genesis Rabbah 82:11).

בְּנֵ֣י לֵאָ֔ה בְּכ֥וֹר יַעֲקֹ֖ב רְאוּבֵ֑ן וְשִׁמְעוֹן֙ וְלֵוִ֣י וִֽיהוּדָ֔ה וְיִשָּׂשכָ֖ר וּזְבֻלֽוּן׃

the sons of Le᾽a; Re᾽uven, Ya῾aqov’s first-born, and Shim῾on and Levi, and Yehuda, and Yissakhar and Zevulun.