'Ammar bin Yasir (RAA) narrated, 'Whoever fasts the day of doubt he has then disobeyed Abu al-Qasim (the Prophet).' Related by the five Imams, and Ibn Khuzaimah and Ibn Hibban graded it as Sahih (RU)
English reference (RU): Book 5, Hadith 672 (RU)
Arabic reference (RU): Book 5, Hadith 651 (RU)