We have to understand the pasuk that says that says "ואתה דבר אל בני ישראל" - And YOU speak to the sons of Israel. What is the emphasis "ואתה" - "And you". We also have to understand what it means when it says "ואתה דבר אל בני ישראל לאמור" - normally "לאמור" means to tell others but here the commandment is to each and every Jew about Shabbos, so who are they supposed to tell further??
[See the Ohr Hachaim and others].
Says the Gemara [Shabbos 10b]:
ואמר רבא בר מחסיא אמר רב חמא בר גוריא אמר רב הנותן מתנה לחבירו צריך להודיעו שנאמר לדעת כי אני ה׳ מקדשכם תניא נמי הכי לדעת כי אני ה׳ מקדשכם אמר לו הקדוש ברוך הוא למשה מתנה טובה יש לי בבית גנזי ושבת שמה ואני מבקש ליתנה לישראל לך והודיעם מכאן אמר רבן שמעון בן גמליאל הנותן פת לתינוק צריך להודיע לאמו מאי עביד ליה אמר אביי שאיף ליה משחא ומלי ליה כוחלא והאידנא דחיישינן לכשפים מאי אמר רב פפא שאיף ליה מאותו המין
Rava bar Mechasseya said that Rav Chama bar Gurya said that Rav said: One who gives a gift to another must inform him that he is giving it to him. As it is stated: “Only keep My Shabbasos for it is a sign between Me and you for your generations to know that I am God Who sanctifies you”(Exodus 31:13). When the Holy One, Blessed be He, gave Shabbos to Israel, He told Moshe to inform them about it. That was also taught in a braisa: The verse states: “For I am God Who sanctifies you,” meaning that the Holy One, Blessed be He, said to Moshe: I have a good gift in My treasure house and Shabbos is its name, and I seek to give it to Israel. Go inform them about it. From here Rabban Shimon ben Gamliel said: One who gives a gift of bread to a child needs to inform his mother that he gave it to him. The Gemara asks: What does he do to the child, so that his mother will know that he gave him a gift? Abaye said: He should smear him with oil or place blue shadow around his eye in an obvious manner. When the mother of the child notices and asks him about it, he will tell her that so-and-so gave him a piece of bread. The Gemara asks: And now that we are concerned about witchcraft involving oil or eye shadow, what should one who gives a gift do? Rav Pappa said: He should smear him with food of the same type that he gave him to eat.
איני והאמר רב חמא (בר) חנינא הנותן מתנה לחבירו אין צריך להודיעו שנאמר ומשה לא ידע כי קרן עור פניו בדברו אתו לא קשיא הא במילתא דעבידא לאגלויי הא במילתא דלא עבידא לאגלויי והא שבת דעבידא לגלויי מתן שכרה לא עביד לגלויי
With regard to the halacha itself, the Gemara asks: Is that so? Didn’t Rav Chama bar Chanina say: One who gives a gift to his friend need not inform him, as God made Moshes’ face glow, and nevertheless it is stated with regard to Moshe: “And Moshe did not know that the skin of his face shone when He spoke with him” (Exodus 34:29)? The Gemara answers: This is not difficult. When Rav Ḥama bar Chanina said that he need not inform him, he was referring to a matter that is likely to be revealed. When Rabban Shimon ben Gamliel said that he needs to inform him, he was referring to a matter that is not likely to be revealed. The Gemara asked: If so, isn’t Shabbos likely to be revealed, as it will be necessary to inform them of Shabbos together with the other mitzvot? Why was Moshe asked to inform them about Shabbos separately? The Gemara answers: The giving of its reward is not likely to be revealed, and it was necessary to inform them about so extraordinary a gift.
We have to understand this Gemara. Why does it say that the reward of SHABBOS is not known implying that the reward of other mitzvos is known when the reality is that the reward of all mitzvos is not known??! Yet, we don't find about other mitzvos "לך והודיעם" - to go and tell them. It says in Pirkei Avos [2-1] that we don't know the reward of mitzvos, so how is Shabbos different that Moshe was commanded to tell them about the reward? [See Maharsha in Mahadura Basra and Iyun Yaakov who allude to this].
The answer is [alluded to by the Ohr Hachaim and see also the Malbim] based on the Mechilta on the pasuk "כי אני ה' מקדשכם" - "to know that I Hashem sanctify you". This refers, says the Mechilta, to Olam Haba which can be tasted in Shabbos in this world. We thus learn that the Kedusha of Shabbos is similar to the Kedusha of Olam Haba. So too the pasuk says "מזמור שיר ליום השבת - לעולם שכולו שבת". Our Shabbos is like the eternal Shabbos. End quote from Mechilta. See Torah Shleima [אות מ"ג] for numerous sources that Shabbos is מעין עולם הבא. THAT is what the Gemara means when it says "מתנה טובה יש לי בבית גנזי - I have a good gift in my treasure house" - "treasure house" refers to Olam Haba because Shabbos is not from this world but from the next.
See the Maharsha in Shabbos who asked about the hava amina of the Gemara that the discussion is about Shabbos itself and not the reward. According to that, what does "בית גנזי" mean? See there where he says that the work of the six days is included in the curse of Adam Harishon "בעצבון תאכלנה" and therefore עכו"ם ששבת חייב מיתה. THAT is the בית גנזי before the sin of Adam when he had rest all 6 days. This is like we said that the matter of resting on Shabbos is not relevant to this world and rather comes from Olam Haba and Adam was in the supernal world of Gan Eden. Because of this it is considered "בית גנזי". The same applies to the conclusion that we are talking about reward - מתן שכרה. The reward for other mitzvos is only in the world to come. But Shabbos, even though it comes from Olam Haba, is written in the Torah here. So too, the reward was stated in this world, because Shabbos is when the next world is already given in this world. From this we can learn about the reward in Olam Haba for other mitzvos, as the Mechilta implies. For Shabbos was given as a taste of the next world in this world and from this we can know at least a little about the reward of mitzvos in the next world.
