I read that the Messiah will not only be a descendant of King David, but also of his son Solomon. What is the source for this?
Let’s start with King David.
Various verses throughout Scripture clearly state that the Messiah, known in Hebrew as the Moshiach, will be a descendant of the house of David:
And a shoot shall spring forth from the stem of Jesse [David’s father], and a twig shall sprout from his roots. (Isaiah 11:1)
Behold, days are coming, says the L‑rd, when I will set up of David a righteous shoot, and he shall reign as king and prosper, and he shall perform judgment and righteousness in the land. In his days, Judah shall be saved and Israel shall dwell safely, and this is his name that he shall be called, The L‑rd is our righteousness. (Jeremiah 23:5–6)
The reason that the Moshiach will be a descendant of King David is because, once David was anointed king by the prophet Samuel, he acquired the kingship for himself and his descendants forever. The verse thus states (II Samuel 7:16), “Your throne shall be established forever.” This acquisition was conditional, applying only to his righteous descendants, as the verse in Psalms states (132:12), “If your children will keep My covenant . . . their children shall also sit on your throne forever.”
Nevertheless, G‑d assured David that the monarchy would not be taken from his descendants forever:
If his children will forsake My Torah and cease walking in My statutes . . . I will punish their transgressions with the rod, and their sins with plagues. Nevertheless, I will not utterly remove My grace from him . . . His throne shall be . . . established forever. (Psalms 89:31–38)
While this promise seems to be made about all of David’s offspring, we find that G‑d singles out King Solomon (I Chronicles 22:9–10):
Behold, a son will be born to you; he will be a man of peace, and I shall give him peace (shalom) from all his enemies around about, for Solomon (Shlomo) will be his name, and I shall give peace and quiet to Israel in his days. He shall build a house in My name, and he shall be to Me as a son, and I to him as a father, and I shall prepare the throne of his kingdom forever.
And later on, David states:
And of all my sons—for the L‑rd gave me many sons—He chose my son Solomon to sit on the throne of the kingdom of the L‑rd over Israel . . . (Ibid. 28:5)
When King David reaffirms that Solomon will reign after him, he is saluted with the expression, “Let my lord King David live forever” (I Kings 1:31), indicating that the eternal monarchy continues through Solomon.
In light of the above, the fact that Moshiach will be a descendant of both David and Solomon is part of the twelfth (of the thirteen) Jewish fundamental beliefs as outlined by Maimonides.
However, it is interesting to note that while it is clear from all of the above sources that the Messiah will be a descendant of King Solomon, the Zohar seems to state that Moshiach will actually be a descendant of Nathan, a different son of David. Expounding on the verse (Isaiah 40:9), “Upon a lofty mountain ascend, you who brings good tidings to Zion,” the Zohar states:
“You who bring good tidings to Zion” is Hephzibah, the wife of Nathan son of David, who is the mother of Messiah, Menachem son of Amiel. She shall go out and bring the tidings . . .”
The famed 20th-century Jewish scholar and kabbalist Rabbi Reuven Margolies explains that the Zohar is careful to describe the Moshiach as being a descendant of Nathan’s wife, rather than of Nathan himself. Nathan had passed away childless, and Solomon his brother married his widow, according to the laws of yibum, levirate marriage.
In a levirate marriage, the firstborn son of the widow and the brother of the deceased is considered to be a continuation of the dead husband’s line. Therefore, Moshiach is referred to here as “offspring” of Nathan, even though he is a descendant of King Solomon.