Why does the Torah give two reasons for observing Shabbos, one in Exodus(20:10: “because G-d made the heavens and the earth in six days, and on the seventh day He rested”) and one in Deuteronomy (5:15: “because you were a slave in Egypt”)?
The version of the Ten Commandments in Exodus is what G-d gave Moses on Mount Sinai; the version in Deuteronomy is part of Moses’ farewell address to the people. It makes sense that the first version should emphasize the creation-related motive, and the second version should emphasize the humanitarian one; in fact, the Midrash tells us that the young Moses persuaded Pharaoh to give the Jewish slaves a day off every week. Both versions are important; as the Midrash tells us, “shamor” (the first word of the fourth commandment in Deuteronomy) and “zachor” (its first word in Exodus) were said simultaneously — they are aspects of the selfsame commandment.