An Arabi came to the Messenger of Allah ﷺ with a roasted rabbit and placed it before him. The Messenger of Allah ﷺ held back and did not eat, but instructed the people to eat. The Arabi also held back, so the Prophet ﷺ asked him, "What keeps you from eating?"He replied, "I am fasting three days of the month."The Prophet ﷺ said, "If you are going to fast, then fast the bright days."
أخبرنا محمد بن معمر قال حدثنا حبان قال حدثنا أبو عوانة عن عبد الملك بن عمير عن موسى بن طلحة
جاء أعرابي إلى رسول الله ﷺ بأرنب قد شواها فوضعها بين يديه فأمسك رسول الله ﷺ فلم يأكل وأمر القوم أن يأكلوا وأمسك الأعرابي فقال له النبي ﷺ ما يمنعك أن تأكل قال إني صائم ثلاثة أيام من الشهر قال إن كنت صائما فصم الغر
[Sahih] Ahmad narrated it, 2/336, 346, from the hadith of Abu Awana, with this wording. It is also in Al-Kubra, no. 2729, and Ibn Hibban graded it sahih, no. 945. Abdul-Malik bin Umayr transmitted it with "an" (without specifying direct hearing), and the hadith has corroborating narrations. No. 2729.