Parshas Vaeschanan

In the Ten Commandments in this weeks parsha, it says ‘zachor es yom hashabbos likodsho kaasher tzivcha’, remember the Shabbos as I commanded you. Rashi explains that the words ‘kaasher tzivcha’ refers to the command of Shabbos they received in Mara. A question arises, why in the luchos rishonim does it not say ‘kaasher tzivcha’, that it had been commanded in Mara?

Here the posuk says 'shamor'watch which is referring to the negative commandments of shabbos,be careful to watch the shabbos not to desecrate it. Previously it says 'zachor' rememeber, chazal say remember it with kiddush this is refering to the positive side iof   shabbos, rememeber to keep to keep shabbos posituively.  The shabbos of Mara which was before the giving of the torah was  a concept of do not desecrate the shabbos therefor in the posuk of watching it reminds us about Mara. The possitive side4 of shabboss was only after Bnei Yisrael recevied the torah and not by Mara so the posuk doesn't say it.