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