Sunan an-Nasa'i, Volume 5, Book 42, Hadith 4315 (DarusSalam) ⓘ

It was narrated that Abu Hurairah said: "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)."

أَخْبَرَنَا مُحَمَّدُ بْنُ مَعْمَرٍ الْبَحْرَانِيُّ، قَالَ حَدَّثَنَا حَبَّانُ، - وَهُوَ ابْنُ هِلاَلٍ قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ عَبْدِ الْمَلِكِ بْنِ عُمَيْرٍ، عَنْ مُوسَى بْنِ طَلْحَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ صلى الله عليه وسلم بِأَرْنَبٍ قَدْ شَوَاهَا فَوَضَعَهَا بَيْنَ يَدَيْهِ فَأَمْسَكَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَلَمْ يَأْكُلْ وَأَمَرَ الْقَوْمَ أَنْ يَأْكُلُوا وَأَمْسَكَ الأَعْرَابِيُّ فَقَالَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ مَا يَمْنَعُكَ أَنْ تَأْكُلَ ‏"‏ ‏.‏ قَالَ إِنِّي أَصُومُ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ ‏.‏ قَالَ ‏"‏ إِنْ كُنْتَ صَائِمًا فَصُمِ الْغُرَّ ‏"‏ ‏.‏