It was narrated that Abu Hurairah said:

"A Bedouin came to the Prophet with a rabbit that he had grilled it and placed it in front of him. The Messenger of Allah refrained from eating, but he told the people to eat. The Bedouin also refrained, and the Prophet ﷺ said to him: 'What is keeping you from eating?' He said: 'I fast three days of the month.' He said: 'If you was to fast, fast the shining days."'

أَخْبَرَنَا مُحَمَّدُ بْنُ مَعْمَرٍ، قَالَ حَدَّثَنَا حَبَّانُ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ عَبْدِ الْمَلِكِ بْنِ عُمَيْرٍ، عَنْ مُوسَى بْنِ طَلْحَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ جَاءَ أَعْرَابِيٌّ إِلَى رَسُولِ اللَّهِ ﷺ بِأَرْنَبٍ قَدْ شَوَاهَا فَوَضَعَهَا بَيْنَ يَدَيْهِ فَأَمْسَكَ رَسُولُ اللَّهِ ﷺ فَلَمْ يَأْكُلْ وَأَمَرَ الْقَوْمَ أَنْ يَأْكُلُوا وَأَمْسَكَ الأَعْرَابِيُّ فَقَالَ لَهُ النَّبِيُّ ﷺ " مَا يَمْنَعُكَ أَنْ تَأْكُلَ " . قَالَ إِنِّي أَصُومُ ثَلاَثَةَ أَيَّامٍ مِنَ الشَّهْرِ . قَالَ " إِنْ كُنْتَ صَائِمًا فَصُمِ الْغُرَّ " .