Thabit bin Yazid al-Ahwl ثابت بن يزيد الأحول · The Book of Hunting and Slaughtering

"A Bedouin brought a rabbit to the Messenger of Allah that he had grilled and placed it before him. The messenger of Allah refrained from eating but he told the people to eat. The Bedouin also refrained from eating, and the Messenger of Allah said to him: 'What is keeping you from eating?' He said: 'I fast three days of each month.' He Said: "If you are going to fast, then fast the bright days (Al-Ghurr)."