"Vísamti Pidus Ben Ami UíBen Amecha" (Perek Ches Pasuk Yud Tes). Hashem tells Moshe Rabbeinu to go to Paraoh and tell him to let Bnei Yisrael go. Moshe is also told to warn Paraoh that if he does not follow Hashemís wishes, a swarm of wild animals will attack Miztrayim. Hashem warns that this swarm of animals will not attack Bnei Yisrael because Hashem will make a sign. Rashi Hakadosh explains that this sign was to divide between the nation of Hashem and the nation of Paraoh.
What is the sign? The Divrei Yisrael on page 26 of his Sefer on Shemos explains that the sign is the Mitzvah of Shabbos. Shabbos is what divides us from the rest of the world -- "HaMavdil Ben Yisrael LíAmim."
Hashem tells Moshe "LíMachar Yiheyeh Haos Hazeh," this sign will be visible tomorrow. Shabbos is "Aos He LíOlam" an eternal sign between us and Hashem.
What is interesting is that the gematria of the words "Vísamti Pidus"-- Vov, Sin, Mem, Tof, Yud, Pey, Daled, Tof -- is the same gematria as the words "Mitzvas Shabbos"--,Mem, Tzaddik, Vov, Tof, Shin, Bais, Tof plus one for each word. Shabbos is the eternal division between us and other nations. In times of great tragedy and great triumph it is keeping the Shabbos that makes of worthy of Hashemís attention and love.