Musar zu Schemuel II 22:51

Before explaining the meaning of the "arks," let me mention a comment by the Zohar on Proverbs 18,10: 13 מגדל עוז שם ה' בו ירוץ צדיק ונשגב, "The name of the Lord is a tower of strength; the righteous runs towards it and is elevated." The "tower" is a reference to the ark, the "righteous" is a reference to Noach. While on the subject, I shall also explain the difference between the word Migdol in Samuel II 22,51, and the alternate reading of the same word as Magdil both in Samuel and in Psalms 18,51. We are in the habit of reciting this verse at the end of saying Grace, and our sages suggested that on weekdays we should recite the version Magdil, whereas on Sabbaths and New Moons we should recite the spelling Migdol. There is a further third spelling, Migdal David in Song of Songs 4,4 כמגדיל דויד צוארך, בנוי לתלפיות, "Your neck is the Tower of David, built as a landmark."