Musnad Ahmad 2:129

Abu al-Nadr narrated to us, al-Mas'udi narrated to us, from Hakim ibn Jubayr, from Musa ibn Talhah, from Ibn al-Hutakiyyah, who said: Food was brought to Umar ibn al-Khattab, so he invited a man to it. The man said: "I am fasting." Then Umar said: "And what kind of fasting are you fasting? Were it not for my dislike of adding or subtracting, I would have told you a hadith of the Prophet, peace and blessings of Allah be upon him, when the Bedouin came to him with a rabbit. But send for Ammar." When Ammar came, he said: "Were you a witness to the Messenger of Allah, peace and blessings of Allah be upon him, the day the Bedouin came to him with a rabbit?" He said: "Yes." He said: "I saw blood on it." The Prophet, peace and blessings of Allah be upon him, said: "Eat it." The man said: "I am fasting." He said: "And what kind of fasting are you fasting?" He said: "The beginning of the month and its end." He said: "If you are going to fast, then fast the thirteenth, the fourteenth, and the fifteenth."