Abu Dawud Book 7, Hadith Number 2327.

Narated By Ammar : AbuIshaq reported on the authority of Silah: We were with Ammar on the day when the appearance of the moon was doubtful. (The meat of) goat was brought to him. Some people kept aloof from (eating) it. Ammar said: He who keeps fast on this day disobeys AbulQasim (i.e. the Prophet) (PBUH).