Holy cow

There was a certain pious man who had a cow. The cow worked all week diligently but rested on Shabbat. The pious man had to sell the cow to a non-Jew. The cow continued to work as it always did, but would stop on Shabbat. When her new owner saw that, he went to the pious man. The pious man whispered to the cow's ear that now that she belonged to a non-Jew, she could work on Shabbat, and the cow went to work. The pious man explained to the new owner what happened.

The non-Jew was not satisfied. The cow knew about Shabbat, and he did not. He went to study and converted. His name was Rabbi Yochanan ben Torta (cow in Aramaic).

This cow was a reincarnation of Vashti, the first queen of the Purim story. She used to make Jewish women work on Shabbat. She corrected this in her next life as a cow. In fact, the pious man recognized the fact and sold her because of it. 

The non-Jew who bought the cow was the reincarnation of Ahasverosh, the king in the Purim story. That was his correction (tikkun).

Art: Cows Resting by Rosa Bonheur