Silah bin Zufar said: We were with Ammar bin Yasir (may Allah be pleased with him) when a roasted goat was brought. He said: Eat. Some of the people moved aside, and one said: I am fasting. Ammar bin Yasir (may Allah be pleased with him) said: "Whoever fasts on the day of doubt has disobeyed Abu al-Qasim (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7952