Albert Barnes Commentary 2 Samuel 23:1

"Now these are the last words of David. David the son of Jesse saith, And the man who was raised on high saith, The anointed of the God of Jacob, And the sweet psalmist of Israel:" — 2 Samuel 23:1 (ASV)

The last words of David - That is, his last psalm, his last “words of song” (2 Samuel 22:1). The inclusion of this psalm, which is not in the Book of Psalms, was likely suggested by the inclusion of the long psalm in 2 Samuel 22.

David the son of Jesse said... - The original word for “said” is used hundreds of times in the phrase “saith the Lord,” which designates the word of God in the mouth of a prophet. It is applied to the words of a man only here; in the strikingly similar passages of Numbers 24:3-4 and Numbers 24:15-16; and in Proverbs 30:1. In all these instances, the words spoken are inspired. The description of David is divided into four clauses that correspond to and balance each other.