Midrash for Exodus 31:17

The halakhah is: A Jew who was walking on the way Erev Shabbat and it grew dark, and he had in his hand coins or something else, what should he do? This is what our sages taught? One for whom the path grows dark [erev Shabbat] he should give his wallet to a non-Jew. And why is it permitted to give it to a non-Jew? R. Levi said, when the children of Noah were commanded, they were commanded only about seven things, and Shabbat was not one of them, therefore they allowed it to be given to a non-Jew. And R. Yose bar Hanina said: A non-Jew who keeps Shabbat before he accepts upon himself circumcision is obligated for the death penalty. Why? For they were not commanded to keep this. And why did you says that a non-Jew who keeps Shabbat is liable for the death penalty? R. Hiyya b. Abba said in the name of R. Yohanan: It is the way of the world that a king and a matron are having a discussion, and one who sticks their head in, are they not liable for death? Thus Shabbat is a matter between Israel and God, as it says, "Between me and the children of Israel." [Thus any non-Jew who puts himself between them before he accepts circumcision, is obligated for death.] ...

R. Jochanan, in the name of R. Simon b. Jochai, said: Every commandment which the Holy One, praised be He! gave unto Israel, was given publicly with the exception of the Sabbath, which was given to them in privacy as it is said (Ib. ib., 17) It is a sign between Me and the children of Israel for ever. If so, then why should a non-Jew be punished for failing to observe it? The Sabbath itself indeed He made well known, only the additional soul He left unknown, for Resh Lakish said: "An additional soul is given by the Holy One, praised be He! to a man on every Sabbath eve, and on Saturday night He takes it away from him, as it is said (Ib. ib., ib.), He ceased from work and rested; i.e., Vayinafash (and rested) means Vy-nefesh (woe soul)."

Our Rabbis were aught: The men of the watch used to pray that the sacrifices of their brethren should be favorably accepted; and the standing men (guards) used to congregate in the synagogues and fast four consecutive days; Monday, Tuesday, Wednesday and Thursday. On Monday, [they would fast] on behalf of those who plied the seas; on Tuesday, on behalf of those who traversed the desert; on Wednesday, on behalf of the children that they might be saved from the disease of croup; and on Thursday, on behalf of the pregnant and nursing women, that the former might be happily delivered and the latter retain their strength to nurse their children. On the day preceding the Sabbath they would not fast, in honor of the Sabbath, and so much the more not on the Sabbath itself. Why did they not fast on Sunday? R. Samuel b. Nachmeni said: "Because that is the third day after man was created." Resh Lakish, however, said: "Because of the additional soul that is given to man on the Sabbath, and which leaves him at the close of the Sabbath, for R. Simon b. Lakish said: 'An additional soul is given by the Holy One, praised be He! to a man on Friday and is taken back on Saturday night, as it is said (Ex. 31, 17) Resteth and ceased from his work. 'Vayinafash' (ceased) means Vay-N efesh (woe, soul).'"