The Blood Was a Sign for Israel and the Mercy of One Commandment

(Exodus 12:13) "And the blood shall be for you a sign" - for you a sign, and not for Me a sign. "For you a sign" - and not for others a sign. "On the houses." Why is this said? Has it not already been said, "on the houses in which they shall eat it" (Exodus 12:7)? From that I would know only the houses in which they eat it; from where the houses in which they sleep? The verse states, "on the houses where you are" - in any case. "And I will see the blood." Rabbi Ishmael used to say: But is not everything revealed before Him, as it is said, "He knows what is in the darkness, and the light dwells with Him" (Daniel 2:22), and it says, "Even the darkness is not too dark for You" (Psalms 139:12)? Then why does the verse say, "And I will see the blood"? Rather, as the reward of one commandment that you perform, I reveal Myself and have compassion upon you, as it is said, "And I will pass over you" (Exodus 12:13). "Passing over" means nothing but sparing, as it is said, "As birds flying, so will the LORD of hosts protect Jerusalem; He will deliver it as He passes over and rescues it" (Isaiah 31:5).