שָׁמוֹר וְזָכוֹר בְּדִבּוּר אֶחָד נֶאֶמְרוּ
שָׁמוֹר אֶת יוֹם הַשַּׁבָּת לְקַדְּשׁוֹ כַּאֲשֶׁר צִוְּךָ ה' אֱלֹקֶיךָ. (ה יא)
Guard (observe) the Shabbos day to sanctify it, as Hashem your G-d commanded you.
In this week's parshah, Moshe Rabbeinu continues to review with the Yidden all that they experienced in the midbar, and to give them mussar for the past and guidance for the future. Moshe also revised the entire Torah with them. As part of this revision, the Aseres Hadibros, which was the basis of Kabbalas HaTorah, were repeated.
There are various differences in the text of the Aseres Hadibros in the two parshiyos in which they appear. The first time the Aseres Hadibros are recorded in the Torah, is in Parshas Yisro, at the time of Mattan Torah and the second time is here, in Parshas Va'eschanan. The most apparent difference is seen with the commandment of Shabbos. In Parshas Yisro the Torah says זָכוֹר אֶת יוֹם הַשַּׁבָּת לְקַדְּשׁוֹ - Remember the day of Shabbos to sanctify it, whereas in this week's parshah it says שָׁמוֹר אֶת יוֹם הַשַּׁבָּת לְקַדְּשׁוֹ - Guard the day of Shabbos to sanctify it.
The Gemara (שבועות כ ע"ב) teaches us something astonishing. זָכוֹר and שָׁמוֹר were said [by Hashem] in one utterance; מַה שֶׁאֵין הַפֶּה יָכוֹל לְדַבֵּר וְאֵין הָאֹזֶן יָכוֹל לִשְׁמֹעַ [this is] something that the [human] mouth cannot say nor the ear hear.
The Ramban adds that all the other discrepancies in the wording of the Aseres Hadibros are not significant since their meanings are the same. The difference of zachor and shamor though, does add completely different implications. Zachor is the positive commandment of sanctifying the Shabbos, such as by reciting kiddush, whereas shamor refers to the melachos which are prohibited on Shabbos.
This is why Chazal say that both of the directives, שָׁמוֹר and זָכוֹר, must have been included in the original commandment. The two words have very different meanings and both refer to essential aspects of Shabbos observance. Therefore, they must both have been given at Mattan Torah. It must be that זָכוֹר וְשָׁמוֹר בְּדִבּוּר אֶחָד נֶאֶמְרוּ.
(רמב"ן שמות כ ז)
