Sefer Chasidim Siman 66
Can copy this into an AI, or connect the whole library to your assistant. For deeper learning, you can directly use the original seforim, sites, and apps.
לבנה לוקה סימן רע לשונאיהם של ישראל לפיכך צריך להתענות שהרי אדם מתענה על החלום כ"ש בשביל כל ישראל:
Medieval Jewish mysticism, translated by Sholom Alchanan Singer. Northbrook, Ill. Whitehall Co., 1971
An eclipse of the moon is a bad sign for the Jews¹ therefore he should fast, and inasmuch as one fasts because of a (bad) dream,² all the more so for the sake of all Israel³ (as in case of eclipse).
footnotes: ¹ Sukkah 29a. ² Taanith 12b. ³ See S. H. Bologna (ed.), section 230.