Musnad Ahmad 30:1440

'Affan narrated to us, Abu 'Awanah narrated to us, 'Abd al-Malik ibn 'Umayr narrated to us, from Musa ibn Talhah, from Abu Hurayrah, who said: A Bedouin came to the Messenger of Allah, peace and blessings of Allah be upon him, with a rabbit that he had roasted, along with its mustard and its gravy. He placed it before him, but the Messenger of Allah, peace and blessings of Allah be upon him, refrained and did not eat. He commanded his companions to eat, but the Bedouin refrained. The Messenger of Allah, peace and blessings of Allah be upon him, said to him: "What prevents you from eating?" He said: "I fast three days of the month." He said: "If you are to fast, then fast the white days (the 13th, 14th, and 15th of the lunar month)."