Reference for Psalms 18:51

מגדל [מַגְדִּיל֮] יְשׁוּע֪וֹת מַ֫לְכּ֥וֹ וְעֹ֤שֶׂה חֶ֨סֶד ׀ לִמְשִׁיח֗וֹ לְדָוִ֥ד וּלְזַרְע֗וֹ עַד־עוֹלָֽם׃

Great salvation giveth He to His king; and showeth mercy to His anointed, To David and to his seed, for evermore.

ספר השרשים