Rabbi Yochanan said in the name of Rabbi Shimon ben Yochai (Exodus 31:14-17): Every commandment that the Holy One, blessed be He, gave to Israel He gave publicly, except for the Sabbath, which He gave privately, as it is written, "between Me and the children of Israel." If so, let the nations not be punished on its account! He made the Sabbath known to them; its reward He did not make known to them. And if you wish, say: its reward, too, He made known to them, but the additional soul He did not make known to them. For Rabbi Shimon ben Lakish said: The Holy One, blessed be He, gives a person an additional soul on the eve of the Sabbath, and at the close of the Sabbath He takes it from him, as it is said, "He ceased and was refreshed [shavat va-yinafash]" — once he has rested [shavat], woe [vai], the soul [nefesh] is lost.
The Sabbath Given in Secret and the Extra Soul of Rest
Yalkut Shimoni on Torah 390:5
(שמות לא יד-יז) אֲמַר רַבִּי יוֹחָנָן מִשּׁוּם רַבִּי שִׁמְעוֹן בֶּן יוֹחַאי. כָּל מִצְוָה שֶׁנָּתַן הַקָּדוֹשׁ בָּרוּךְ הוּא לְיִשְׂרָאֵל נָתַן לָהֶם בְּפַרְהֶסְיָא, חוּץ מִן הַשַּׁבָּת שֶׁנָּתַן בְּצִנְעָא דִּכְתִיב בֵּינִי וּבֵין בְּנֵי יִשְׂרָאֵל. אִי הָכִי, לָא לֵיעָנְשׁוּ גוֹיִם עֲלָהּ. שַׁבָּת אוֹדְעִינְהוּ, מַתַּן שְׂכָרָהּ לָא אוֹדְעִינְהוּ וְאִיבָּעִית אֵימָא, מַתַּן שְׂכָרָהּ נַמִי אוֹדְעִינְהוּ, נְשָׁמָה יְתֵרָה לָא אוֹדְעִינְהוּ, דַּאֲמַר רַבִּי שִׁמְעוֹן בֶּן לָקִישׁ, נְשָׁמָה יְתֵרָה נָתַן הַקָּדוֹשׁ בָּרוּךְ הוּא לָאָדָם בְּעֶרֶב שַׁבָּת, וּבְמוֹצָאֵי שַׁבָּת נוֹטְלָהּ (הֵימֶנָּה) [הֵימֶנּוּ] שֶׁנֶּאֱמַר שָׁבַת וַיִּנָּפַשׁ כֵּיוָן שֶׁשָּׁבַת וַי אָבְדָה נֶפֶשׁ.