An Arabi came to the Messenger of Allah ﷺ with a roasted rabbit, along with its mustard sauce and seasoning, and placed it before him. The Messenger of Allah ﷺ held back and did not eat, but instructed his companions to eat. The Arabi also held back, so the Messenger of Allah ﷺ asked him, "What keeps you from eating?"He replied, "I fast three days of every month."The Prophet ﷺ said, "If you are going to fast, then fast the bright days."
أتى أعرابي رسول الله ﷺ بأرنب قد شواها، ومعها صنابها وأدمها، فوضعها بين يديه، فأمسك رسول الله ﷺ فلم يأكل، وأمر أصحابه أن يأكلوا، فأمسك الأعرابي، فقال له رسول الله ﷺ: ما يمنعك أن تأكل؟ قال: إني أصوم ثلاثة أيام من كل شهر. قال: إن كنت صائما فصم الأيام الغر.
Narrated by Nasai (2421), Imam Ahmad (8434), and Ibn Hibban (3650), all from the hadith of Abu Awana, from Abdul-Malik bin Umayr, from Musa bin Talha, from Abu Hurairah, who related it. The wording given here is Ahmad's. Its chain of transmission is sahih.The disagreement over Musa bin Talha's version does not cause harm, for both this version and the one that follows have been established as sahih from him. As for what was reported from him as a mursal narration, it does not invalidate what was reported from him as a connected narration.His saying "the bright days" refers to the white days, and the term covers the nights as well. They are the thirteenth, fourteenth, and fifteenth of every month. They are called white because the moon is full on these nights, so the night is like the day.His saying "sinabuha" refers to mustard prepared with oil, a condiment used for seasoning food. His saying "and its relish" refers to the udm or idam, which is whatever is eaten together with bread, whatever that may be.