John Gill Commentary 1 Samuel 15:28

"And Samuel said unto him, Jehovah hath rent the kingdom of Israel from thee this day, and hath given it to a neighbor of thine, that is better than thou." — 1 Samuel 15:28 (ASV)

And Samuel said to him, the Lord has rent the kingdom of
Israel from you this day
Seeing his mantle rent by Saul, he took occasion from thence to predict, and no doubt it was impressed on his mind by the Spirit of God, that his kingdom should be in a like manner rent from him, on account of his own evil conduct and behaviour; and from this day forward he might expect it; the sentence was gone forth from God, and it would not be reversed; and by a like sign was signified the rending of the ten tribes from the kingdom of Solomon in his son Rehoboam, (1 Kings 11:30 1 Kings 11:31) ,

and has given it to a neighbour of yours, that is better than you ;
who was David, a man after God's own heart, that would fulfil his will, who was more holy, just, and wise than Saul; whose works were better and righter than his, as the Targum; who was an Israelite, of the same nation and religion as he, and so his neighbour; and though he was not of the same tribe, yet of a neighbouring tribe; Benjamin, and Judah, of which tribe David was, joining closely to one another.

It is highly probable that at this time Samuel knew not personally who he was that was designed to be made king in his room, though under the direction of the Spirit of God he thus describes him; for after this he is bid to go to Jesse's family, from thence to anoint a king, and several passed before him before the Lord pointed out the proper person to him.