Psalms 89:3

I have made a covenant with my chosen - With my chosen one; that is, with David. The original is in the singular number, though by the Septuagint, and the Vulgate, and by Luther, it is rendered in the plural - chosen ones - elect. This is undoubtedly the language of God himself, though it is not expressly ascribed to him. The design is to describe the solemn promise which God had made to David and to his posterity. Compare Psalms 78:70. See also, on the use of the phrase “made a covenant,” see Psalms 50:5, note; Psalms 83:5, note.

I have sworn unto David my servant - I have taken a solemn oath in regard to him. The substance of the oath is stated in the next verse. The promise referred to is found in 2 Samuel 7:11.