Reference for Ketubot 207:2

ההוא יומא דנח נפשיה דרבי גזרו רבנן תעניתא ובעו רחמי ואמרי כל מאן דאמר נח נפשיה דר' ידקר בחרב

On the day when Rabbi died the sages decreed a public fast and offered prayers for heavenly mercy. They announced that whoever said that Rabbi was dead would be stabbed with a sword.

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