Musnad Abu Ya'la al-Mawsili 105:11

It was read to Bishr ibn al-Walid while I was present: Abu Yusuf narrated to us, from Abu Hanifah, from Musa ibn Talhah, from Ibn al-Hawtakiyyah, from Umar, that a man asked him about eating a rabbit. He said, "Call Ammar for me." Ammar came, and he said, "Narrate to us the hadith of the rabbit on the day we were with the Messenger of Allah, may Allah bless him and grant him peace, in such and such a place." Ammar said, "A Bedouin gifted a rabbit to the Messenger of Allah, may Allah bless him and grant him peace, and he ordered the people to eat. A Bedouin said, 'I saw blood.' He said, 'It is nothing.' Then he said, 'Draw near and eat.' He said, 'I am fasting.' He said, 'Fasting what?' He said, 'I fast three days of every month.' He said, 'Why did you not make them the white [days]?'"