1 Samuel 17:55

And when Saul saw David go forth against the Philistine, he said unto Abner, the captain of the host, Abner, whose son is this youth? And Abner said, As thy soul liveth, O king, I cannot tell.

Saul ... said unto Abner ... whose son is this youth? A young man is more spoken of in many Eastern countries by his father's name than his own. The growth of the beard and other changes on a now full-grown youth prevented the king from recognizing his former favourite minstrel.