ה֥וּא יִבְנֶה־בַּ֖יִת לִשְׁמִ֑י וְכֹנַנְתִּ֛י אֶת־כִּסֵּ֥א מַמְלַכְתּ֖וֹ עַד־עוֹלָֽם׃
Er wird meinem Namen ein Haus bauen, und ich werde den Thron seines Königreiches gründen auf ewig.
Rashi
For I said, “Forever will it be built with kindness” For I said, “Forever the throne of David will be built with kindness,” as it is said (II Sam. 7:13): “and I shall establish the throne of his kingdom, etc.” Another explanation: I thought that the world would be built with Your kindness and that You would establish Your faithfulness in the heavens, to be established and made permanent. Now what is the faithfulness? That promise You promised David through Nathan the prophet, saying, “I formed a covenant with My chosen one to establish his seed until eternity.”
Rashi
For You have preceded him with the blessings of the good man Before I asked You, You preceded me with Your blessing through Nathan the prophet (in II Sam. 7:12f.): “then I will raise up your seed...and I will establish the throne of his kingdom forever.”
Abarbanel
The first introduction in the matters of the kings memorialized in this book and it appropriate that we discuss ten topics: The first topic is the ??? according to their dates. For of the kings mentioned in these books, only a few ruled the whole nations: Shual, David, and Solomon who reigned over all the tribes united. You will see that all three were Men of Valor, G-d fearing and Men of Truth. As I explained in the Book of Samuel that the reigns summed to 100 years, keep this memory aid to hand (Kings 1, 20:31) "Magnanimous kings are they" for 'Melchi' adds to 100. These kings were unique in reigning over all the tribes without divisions because once the kingdom was divided in the days of Rechavem and Yeravam. King Rechavam and all the progeny of House of David ruled over the tribes of Yehuda and Binyamin, majority of the tribe of Levi and the fraction of the tribe of Shimon whose patrimony was amid the patrimony of the sons of Yehuda. You will see in their stories, which of them walked in peace and wealth along the path of their father, David and which among them ?betrayed? G-d and went wayward to serve other gods. The length of the reign of the Kings of Judah, good and bad, was 393 years. A hint to this (Proverbs 15:3) "The eyes of G-d observe both bad and good, since the words 'raim and tovim' sum to 393 [translator's note: the gemateria is 392 +1 for the phrase]
Rashi
length of days to his kingdom, for You said, “and I will establish the throne of your (sic) kingdom forever” (II Sam. 7:13).
Rashi
Because [my kingdom] will never be outgrown. There will be no other king after my kingdom.8As God promised Dovid earlier in 7:13 that his kingdom will be eternal. Another interpretation: He has made an eternal convenant for my House, that my kingdom shall be everlasting and this convenant is known to all, it is mentioned by all the prophets.