If a man is bitten by a snake and wants to be saved from the snake’s venom, he must slaughter a white she-ass, butcher it, and place the pieces upon his body. The sages added that the she-ass must not be tereifah (she must bear no disfigurement which would cause her death within the year). There was an incident involving the governor of the Babylonian city of Pumbedita, a religious Jew, who was bitten by a snake. They slaughtered 13 white she-asses to save him, but unfortunately all were diseased and disfigured, tereifah. There was one last she-ass on the other side of town, but before they could reach it a lion came along and ate the she-ass. The sage Abaye said that since the Jewish governor could not be saved, he must have transgressed rabbinic edicts. It is written, “whoever breaks through a fence will be bitten by a serpent” (Ecclesiastes 10:8), meaning that one who violates rabbinic edicts and breaks through the fences will be bitten by a snake and will not emerge alive. The scholars told Abaye that his words were correct, that this governor had violated a rabbinic edict. It had been the custom go out into the city street with myrtle branches to gladden brides and grooms, but when the greatest man of the generation, Rav, died, one of the sages, Rav Isaac son of Bisna, decreed that as a sign of sadness and mourning this should not be done for an entire year. This governor violated the sage’s edict and brought myrtle branches to gladden a bride and groom, so a snake bit him and he could not be saved.
(Babylonian Talmud, Tractate Shabbat 109b-110a)