כל חלב וכל דם לא תאכלו (ויקרא ג:יז)
The commandment of not eating any blood.
There was once a blood libel against the Yidden living in a European village, accusing the townspeople of murdering a Christian child to use the blood for their matzahs. The judges summoned the town’s Rav to represent the community at a trial.
The Rav requested that the judges summon a simple Jewish homemaker from the town. When she arrived, the Rav asked the woman to prepare him an egg omelet in a pan. The woman cracked open the egg, and before placing the egg in the pan, she carefully examined the egg to check for any traces of blood.
“Why did you check the eggs?” The Rav asked the woman in the presence of the judges, “What do you mean?” the woman replied simply. “To check that the egg shouldn’t have any traces of blood, G-d forbid.”
The judges were stunned by her response and realized that her statement was an affirmation that if a Jew is so careful to avoid a trace of blood, they won’t use blood for their matzahs.