Where does this Martin Luther quote come from? "Sin is essentially a departure from God." -Martin Luther

Which work does this quote come from?
According to God is for Us: 52 Readings from Romans, by Simon Ponsonby: "During the Diet of Augsburg Luthor wrote to Melancthon, 'Sin is essentially a departure from God.'" Quote is credited in that book to Watchwords for the Warfare of Life, Vol. 1, T. Nelson and Sons.

eBook URL: God is for Us