Why did the Torah write "Zachor" in Yisro and "Shamor" here?
Rashi: Hashem said both words simultaneously as one, and that is how they were heard. 1
Moshav Zekenim: (6:7): "Zachor" refers to the creation, when Hashem created the world in six days [and rested on the seventh - in memory of which we too should rest on the seventh day.
Emes le'Ya'akov: Some say that the first Dibros were written on the first Luchos, and the second Dibros were written on the second Luchos. 2
Oznayim la'Torah #1: "Zachor" refers to the Mitzvos Asei of Shabbos such as Kidush, which the Torah mentioned at Har Sinai, during their sojournings in the desert, when they did not need to do Melachah during the week and the chief aspect of Shabbos was Kidush and the positive Mitzvos connected with Shabbos; whereas "Shamor" refers to the Mitzvos Lo Sa'aseh, such as the thirty-nine Melachos, which the Torah mentions here after they have conquered the land of Sichon and Og and are poised to enter Eretz Cana'an, where the chief aspect of Shabbos is desisting from performing Melachos. 3
"Oznayim la'Torah: "Shamor" has connotations of waiting during the week for Shabbos to arrive 4 - and the Torah adds the word "Lekadsho" - to await the arrival of Shabbos in order to sanctify it, with Torah, Tefilah and reflection. 5
This is not like Rashi (Sanhedrin 56b), who explains that everything in the second Dibros was written on [both sets of] Luchos! Rather, it was like a K'ri and K'siv. (Every difference between the sets of Dibros, one way was written in the Luchos, and there was a tradition for the other way).
Oznayim la'Torah: And it is for the same reason that the Torah inserts "Shorcha" (regarding plowing) and "Chamocha" (regarding transporting manure) in Pasuk 14, which it omitted in Yisro.
Oznayim la'Torah: As in Vayeishev, Bereishis, 37:11 "ve'Aviv Shamar es ha'Davar".
What is "Ka'asher Tzivcha" referring to?
Rashi (citing Shabbos, 87b): It refers to Marah, 1 where Hashem first commanded Yisrael to keep Shabbos. 2
Seforno: It is referring to keeping Shabbos in the way that the Torah indicated in Marah, where it stated "ve'ha'Yashar be'Einav Ta'aseh, ve'Ha'azanta le'Mitzvosav", 3 an indication that one should spend Shabbos studying Torah and Mitzvos. 4
Rashbam: It is referring to the reason that Hashem stated at Sinai (when He gave Moshe the first Luchos
Refer to Sh'mos, 15:25:3:1. See also Torah Temimah, note 4. The question arises why we say in Shabbos Musaf 'Az mi'Sinai Nitztavu Alehah' and not 'Az mi'Marah'? Perhaps at Marah the command was not for all generations. (PF), or because they were only commanded about certain aspects of Shabbos at Marah
QUESTIONS ON RASHI
Rashi writes that Hashem first commanded Yisrael about Shabbos in Marah. What is the source? We find that Moshe told them about Shabbos when a double portion of Manna fell, in Midbar Sin (Sh'mos 16:23)?
