Musnad Ahmad 30:1314

Abu al-Walid narrated to us, Abu 'Awanah narrated to me, from 'Abd al-Malik ibn 'Umayr, 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 sauce 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 every month." He said: "If you are to fast, then fast the white days."