It was ruled in Halacha that one may not chop vegetables on the Sabbath for cooking after the Sabbath is done, but one may chop vegetables on Yom Kippur for cooking after the fast. The question was raised: Is one permitted to chop vegetables on Yom Kippur which falls on a Sabbath? According to one sage it is permitted, and according to another it is forbidden. What are the issues involved in permission or prohibition? The interpretation of the Scriptural word Shabbaton in the verse “Then he said to them, ‘This is what the Lord has said: “Tomorrow is a Shabbaton, a holy Sabbath”‘” (Exodus 16:23). According to the sage who forbids it, the Scriptures order us to refrain from chopping vegetables on the Sabbath, and since one is forbidden by the Torah to chop vegetables, it is prohibited even on a Sabbath which happens to be Yom Kippur. According to the sage who permits it, the Scriptures order us to refrain from work prohibited on the Sabbath; if the prohibition was already given, the Scriptures reinforce the prohibition. Since the prohibition against chopping vegetables is rabbinic and not from the Scriptures, they permitted it on Yom Kippur so that food would be ready immediately after the fast. There was an incident involving the members of Rav Judah’s household, who chopped cabbage on Yom Kippur, long before the fast was over. Thus, too, the members of the sage Rava’s household, who would grate pumpkin on Yom Kippur long before the fast was over. Since the permission to chop vegetables on Yom Kippur applies only close to the end of the fast and not long before, the sage told his household members that he had received a letter from Rabbi Yochanan, a sage who lived in Eretz Israel, stating that one may not chop vegetables on Yom Kippur until it is close to the end of the fast. Even though no such letter had come, he said that it had so that the members of his household would accept his words.
(Babylonian Talmud, Tractate Shabbat 114b-115a)