Deuteronomy 5:12

Sons of Israel, My people, observe the day of Sabbath, to sanctify it according to all that the Lord your God hath commanded.

And now he begins the review of the Torah. He told them the Ten Commandments, not changing and not explaining anything in [the first two commandments]: I am the Eternal etc., and Thou shalt have no other gods before Me, for they heard them from the mouth of the Almighty. So, too, [there are no changes or explanations] in the third commandment etc. because he already stated [here, as in the text of the third commandment in Exodus 20:7, Thou shalt not take] the Name of the Eternal thy G-d [etc.], as I have explained in the section Ye shall be holy. Then he began to exposit to them, saying ‘Observe’ [the Sabbath-day] instead of ‘Remember’ [the Sabbath-day], this being in line with the plain meaning of Scripture [i.e., that ‘Observe’ is the explanation of ‘Remember’]. And in the opinion of our Rabbis “[Remember and Observe] both were spoken in one utterance.” He explained this to them [here] because they all know Remember, and he is thus informing them [now] that Observe was said with it. And so it is by way of the Truth, [the mystic teachings of the Cabala], that they were both spoken in one utterance seeing that the Voice was both in “remembering” and “observing,” for the Torah is an everlasting covenant, ordered in all things, and sure, and he [i.e., Moses] heard from [the attribute of] Remember while they [i.e., the people, heard] from that of Observe. The student [learned in the mystic teachings of the Cabala] will know this from what we have hinted there.
He explained [with reference to the commandment that we are to Observe the Sabbath-day, to keep it holy] as the Eternal thy G-d commanded thee — “before the Giving of the Torah, at Marah, [for there it is said], there He made for them a statute and an ordinance.” This is the language of Rashi. But if so, the commandment to honor father and mother was also given at Marah [for in the commandment, to honor parents, too, it is stated as the Eternal thy G-d commanded thee], but the Rabbi [Rashi] mentioned there [in connection with the verse referring to Marah] only “the Sabbath, the Red Heifer, and the laws of justice” [as having been given at Marah, but not the honoring of parents].
It appears to me by way of the Truth, [the mystic teachings of the Cabala], concerning this matter [of the Ten Commandments] as well as the whole Book of Deuteronomy that Moses was speaking as if from the mouth of the Almighty, as he said, And it shall come to pass, if ye shall hearken diligently unto My commandments which I command you this day, to love the Eternal your G-d, and to serve Him with all your heart and with all your soul, that I will give rain for your Land in its season. Now Moses is not the one who giveth rain upon the earth, nor the one who gives grass in the fields! Rather, the communication of the Torah was given with Remember, and that of Deuteronomy with Observe. Therefore, Moses everywhere in Deuteronomy mentions the Eternal thy G-d while in [the first four books of] the Torah he mentions only the Proper Name of G-d [i.e., the Tetragrammaton] except in the Ten Commandments as it is said, And G-d spoke etc. Understand this.

שמור את יום השבת, I have already explained in Exodus 20 why this time the expression שמור is used instead of the expression זכור which the Torah used there.

כאשר צוך ה' אלוקיך, a reference to G’d having explained the rationale for Sabbath observance in the paragraph dealing with it both in Parshat Yitro (Exodus 20,10) as well as in Parshat Ki Tissa, and Parshat Vayakhel. The expression כאשר צוך used in connection with a positive commandment means that failure to observe it is simultaneously the violation of a negative commandment. We do not find individual negative commandments accompanied by a warning: “which the Lord your G’d has commanded you not to do.”

לקדשו כאשר צוך, in the manner He had already issued commandments pertaining to Sabbath observance at Marah (Exodus 15,26)