Shemot Rabbah
According to Rabbi Levi: If all of Israel were to keep Shabbat properly for even a single day, the Messiah ben David would come. Why? because it is equivalent to all of the commandments, as it is said (Ps. 95:7) "for He is our God, and we are the people He tends, the flock in His care. O, if you would but heed His charge this day:"