God tells Moses that He has a precious gift in His treasury called Shabbat, and He wishes to give it to Israel — Shabbat is described as a matanah tovah, a 'good gift.'
וְאָמַר רָבָא בַּר מַחְסֵיָא אָמַר רַב חָמָא בַּר גּוּרְיָא אָמַר רַב: הַנּוֹתֵן מַתָּנָה לַחֲבֵירוֹ, צָרִיךְ לְהוֹדִיעוֹ. שֶׁנֶּאֱמַר: ״לָדַעַת כִּי אֲנִי ה׳ מְקַדִּשְׁכֶם״. תַּנְיָא נָמֵי הָכִי: ״לָדַעַת כִּי אֲנִי ה׳ מְקַדִּשְׁכֶם״, אָמַר לוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא לְמֹשֶׁה: מַתָּנָה טוֹבָה יֵשׁ לִי בְּבֵית גְּנָזַי וְשַׁבָּת שְׁמָהּ, וַאֲנִי מְבַקֵּשׁ לִיתְּנָהּ לְיִשְׂרָאֵל, לֵךְ וְהוֹדִיעָם. מִכָּאן אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל: הַנּוֹתֵן פַּת לְתִינוֹק, צָרִיךְ לְהוֹדִיעַ לְאִמּוֹ. מַאי עָבֵיד לֵיהּ? אָמַר אַבָּיֵי: שָׁאֵיף לֵיהּ מִשְׁחָא, וּמָלֵי לֵיהּ כּוּחְלָא. וְהָאִידָּנָא דְּחָיְישִׁינַן לִכְשָׁפִים מַאי? אָמַר רַב פָּפָּא: שָׁאֵיף לֵיהּ מֵאוֹתוֹ הַמִּין. אִינִי וְהָאָמַר רַב חָמָא (בַּר) [בְּרַבִּי] חֲנִינָא: הַנּוֹתֵן מַתָּנָה לַחֲבֵירוֹ אֵין צָרִיךְ לְהוֹדִיעוֹ, שֶׁנֶּאֱמַר: ״וּמֹשֶׁה לֹא יָדַע כִּי קָרַן עוֹר פָּנָיו בְּדַבְּרוֹ אִתּוֹ״! — לָא קַשְׁיָא: הָא בְּמִילְּתָא דַּעֲבִידָא לְאִגַּלּוֹיֵי, הָא בְּמִילְּתָא דְּלָא עֲבִידָא לְאִגַּלּוֹיֵי. וְהָא שַׁבָּת, דַּעֲבִידָא לִגַלּוֹיֵי? — מַתַּן שְׂכָרָהּ לָא עָבֵיד לִגַלּוֹיֵי.
Incidental to the halakhic statement in his name, the Gemara also cites another statement that Rava bar Meḥasseya said that Rav Ḥama 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 Shabbatot 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 Shabbat to Israel, He told Moses to inform them about it. That was also taught in a baraita: The verse states: “For I am God Who sanctifies you,” meaning that the Holy One, Blessed be He, said to Moses: I have a good gift in My treasure house and Shabbat 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. With regard to the halakha itself, the Gemara asks: Is that so? Didn’t Rav Ḥama bar Ḥanina say: One who gives a gift to his friend need not inform him, as God made Moses’ face glow, and nevertheless it is stated with regard to Moses: “And Moses 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 Ḥanina 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 Shabbat likely to be revealed, as it will be necessary to inform them of Shabbat together with the other mitzvot? Why was Moses asked to inform them about Shabbat 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.