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