TT #91 · Parashas Shoftim · Second Aliyah · Passuk 17:15
You shall surely set over yourself a king whom the Eternal your God will choose; from among your brethren you shall set a king over yourself; you may not place over yourself a foreigner who is not your brother.
Torah Temimah points to Yevamos 45b / related.
TT lemma: אֲשֶׁר לֹא אָחִיךָ הוּא.
Sefaria — Yevamos 45b / related
English: A convert who comes from the nations is not fit for kingship until his mother is from Yisrael.
Derash on lineage requirements (Bavli) — high office requires full membership in the people through maternal lineage.
Kingship (and certain parallel offices) requires that the candidate be from ‘your brethren’ in the full sense, including maternal Yisrael status.