Psalms 132:11
The LORD hath sworn in truth unto David; he will not turn from it; Of the fruit of thy body will I set upon thy throne. The LORD hath sworn in truth unto David; he will not turn from it; Of the fruit of thy body will I set upon thy throne.
The Lord swore in truth to David, and He will not turn from His oath. That swearing was swearing by Himself, showing that a solemn oath may affirm what is true. The oath promised that of the fruit of David's body, of his loins according to the flesh, a descendant would be set upon his throne.
That promised descendant is Christ, whom God would raise up to sit on David's throne. David knew this beforehand because he spoke as a prophet seeing the future. God fulfilled the promise by raising Jesus from the dead and exalting Him to reign at God's right hand.
David was not speaking of himself, for he is dead and buried and his sepulchre remains to this day. He spoke of Christ, whose soul was not left in hell and whose flesh did not see corruption, whom God raised up.