John Gill Bible Commentary

Yea, my reins shall rejoice,.... Which is only another phrase expressive of the same thing, and confirming the greatness of joy on the above occasion; not only his heart rejoiced, which was affectionately concerned for his son, near which he lay, the desires of which were frequently drawn out for his good, but his reins also; the seat of the afflictions rejoiced at it; showing how vehement, sincere, and hearty the joy was; when thy lips speak right things; as they will, when the heart is wise; things agreeably to right reason, to the Scriptures of truth, the oracles of God; to the law and to the testimony; to the Gospel of Christ, and the doctrines of it; and such things as are savoury, pleasant, and profitable, and minister grace to the hearers. The Targum is, "when my lips speak right things;'' see Pro 8:6.