Sunan an-Nasa'i 2429

It was narrated that Musa bin Talhah said:"A rabbit that a man had grilled was brought to the Prophet and when he offered it to him he said: 'O Messenger of Allah, I saw some blood on it." The Messenger of Allah did not eat it, but he said to those who were with him: "Eat; if I felt like it, I would have eaten it.' There was a man sitting, and the Messenger of Allah said: 'Come and eat with the people.' He said: 'O Messenger of Allah, I am fasting.' He said: 'Why don't you fast Al-Bid?' He said: 'What are they?' He said: "The thirteenth, fourteenth and fifteenth?