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 me4ssenger of Allah refrained form eating but he told the people to eat. The Bedouin also refrained from eating, and the Mesenger of Allah said to him: 'What is keeping you from eating?" He said: 'I fast three days of e4ach 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 ﷺ بِأَرْنَبٍ قَدْ شَوَاهَا فَوَضَعَهَا بَيْنَ يَدَيْهِ فَأَمْسَكَ رَسُولُ اللَّهِ ﷺ فَلَمْ يَأْكُلْ وَأَمَرَ الْقَوْمَ أَنْ يَأْكُلُوا وَأَمْسَكَ الأَعْرَابِيُّ فَقَالَ لَهُ رَسُولُ اللَّهِ ﷺ " مَا يَمْنَعُكَ أَنْ تَأْكُلَ " . قَالَ إِنِّي أَصُومُ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ . قَالَ " إِنْ كُنْتَ صَائِمًا فَصُمِ الْغُرَّ " .