Charles Ellicott Commentary 2 Samuel 3:18

"now then do it; for Jehovah hath spoken of David, saying, By the hand of my servant David I will save my people Israel out of the hand of the Philistines, and out of the hand of all their enemies." — 2 Samuel 3:18 (ASV)

The Lord has spoken. —The promise here quoted is not contained in so many words in the records which have come down to us. It may have been either an unrecorded utterance of one of the prophets (Samuel, Gad, or Nathan), or simply a reasonable inference from what had been promised, and from the Divine support of David in his career until now.