In Mishneh Torah-The Laws of Kings 4:4 Rambam says:
(The king of Israel) may take wives and pilagshim from the entire territory of Eretz Yisrael. The term 'wives' implies women who were married with a ketubah and kiddushin; pilagshim, women who were not given a ketubah and kiddushin. With the act of yichud alone, the king acquires her and relations with her are permitted him.
What is the legal source (Talmudic or otherwise) used by Rambam to claim that the king of Israel can acquire a pilegesh with yichud alone?