שמור. וּבָרִאשׁוֹנוֹת הוּא אוֹמֵר "זָכוֹר", שְׁנֵיהֶם בְּדִבּוּר אֶחָד וּבְתֵבָה אַחַת נֶאֶמְרוּ, וּבִשְׁמִיעָה אַחַת נִשְׁמְעוּ (מכילתא): שמור OBSERVE [THE SABBATH DAY] — But in the former Ten Commandments (i.e. where they were first promulgated, in Exodus XX.), it states, “Remember [the Sabbath day]"! The explanation is: Both of them (זכור and שמור) were spoken in one utterance and as one word, and were heard in one hearing (i.e. were heard simultaneously) (Mekhilta d'Rabbi Yishmael 20:8:1).
כאשר צוך. קֹדֶם מַתַּן תּוֹרָה בְּמָרָה (שבת פ"ז): כאשר צוך [OBSERVE THE SABBATH DAY TO SANCTIFY IT] AS [THE LORD THY GOD] COMMANDED THEE before the giving of the Law, at Marah (Shabbat 87b).