'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 (JA)
English reference (JA): Book 5, Hadith 672 (JA)
Arabic reference (JA): Book 5, Hadith 651 (JA)