Judah Halevi,
Kitab al Khazari (Kuzari) — Part Two
“ He who observes the Sabbath because the work of creation was finished on it acknowledges the creation itself. He who believes in the creation believes in the Creator. He, however, who does not believe in it falls a prey to doubts of God's eternity and to doubts of the existence of the world's Creator. The observance of the Sabbath is therefore nearer to God than monastic retirement and ascetism. Behold how the Divine Influence attached itself to Abraham, and then to all those who shared his excellence and the Holy Land. ”
