Adam Clarke on 2 Samuel 13:21

But when King David heard - To this verse the Septuagint add the following words: Και ουκ ελυπησε το πνευμα Αμνων του υἱου αυτου, ὁτι ηγαπα αυτον, ὁτι πρωτοτοκος αυτου ην; "But he would not grieve the soul of Amnon his son, for he loved him, because he was his first-born." The same addition is found in the Vulgate and in Josephus, and it is possible that this once made a part of the Hebrew text.