John Gill 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)

Now then do [it] Make him your king, and I shall no longer oppose it as I have done:

for the Lord has spoken of David ; concerning his being king, and the saviour of his people Israel:

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 ; and which, though where recorded in so many words, yet was the sense of the promise of making him king, and the design of his unction.

Besides they might have been spoken to Samuel, though not written; and which he might report, and so might pass from one to another to be generally known.