Various Midrash Quotes
Shabbat
G-d said to the Jewish people "If you observe Shabbat, I will consider it as if you observe all the mitzvot but if you don't observe Shabbat, I will consider it as if you don't observe any of the mitzvot". Midrash Shemot Rabbah 25
If the entire Jewish nation would observe even one Shabbat, the Mashiach would then come. (ibid)