Rashi on Deuteronomy 5:15/רש"י על דברים ה׳:ט״ו

1וזכרת כי עבד היית וגו׳ AND THOU SHALT REMEMBER THAT THOU WAST A SLAVE [IN THE LAND OF EGYPT … AND THE LORD BROUGHT THEE FORTH FROM THERE … THEREFORE HE COMMANDS THEE TO KEEP THE SABBATH DAY] — On that condition He freed you — that you should be a servant to Him and observe His commandments.