Halakhah zu Schemuel II 7:16

And David and his seed have already acquired the monarchy of Israel. And it is not still in our hands to change it - [just] like it is no longer in our hands to change the priesthood from the seed of Aharon - as it is stated about it (II Samuel 7:16), "your throne shall be established forever." And in the elucidation [of this], they, may their memory be blessed, said (Kohelet Rabbah 7; Yoma 72b), [that] the crown of monarchy was acquired by David. And all who believe in the Torah of Moshe concede to this. And the rest of the details of the commandment are in [various] places in Yevamot, Sanhedrin, Sotah and Niddah. And I have already written some of the laws of the monarchy, as is my custom, above in this Order on the commandment of appointing a king.