Why did the Torah write "Zachor" in Yisro and "Shamor" here?
Rashi and Ramban: Hashem said both words simultaneously as one, and that is how they were heard. 1
What is "Ka'asher Tzivcha" referring to?
Rashi (citing Shabbos, 87b): It is referring to Marah, 1 where Hashem first commanded Yisrael about keeping Shabbos.
Seforno: It is referring to keeping Shabbos in the way that the Torah indicated at Marah, where it stated "ve'ha'Yashar be'Einav Ta'aseh, ve'Ha'azanta le'Mitzvosav", 2 an indication that one should spend Shabbos studying Torah and Mitzvos. 3
Rashbam: It is referring to the reason that Hashem stated at Sinai (when He gave Moshe the first Luchos - on which He wrote "Ki Sheishes Yamim Asah Hashem es ha'Shamayim ve'es ha'Aretz". 4
Refer to Sh'mos, 15:25:3:1. The question arises however, as to why we say in Shabbos Musaf 'Az mi'Sinai Nitztavu alehah' and not 'Az mi'Marah ...'?