Musnad Ahmad 210
It was narrated that Ibn al-Hawtakiyyah said:
Some food was brought to 'Umar bin al-Khattab and he called a man to join him, but he said, I am fasting. ['Umar] said: What fast are you observing? Were it not for fear of adding or subtracting something, I would have narrated to you a hadeeth, from the Prophet (ﷺ), when the Bedouin brought him a rabbit; rather send for 'Ammar. When  'Ammar came, he said: Were you there on the day when the Bedouin brought the rabbit to the Messenger of Allah (ﷺ)? ('Ammar) said: Yes. ('Umar] said: I saw blood on it, but he (the Prophet (ﷺ) said: `Eat it.` (The Bedouin) said: I am fasting. (The Prophet (ﷺ) said: `What fast are you observing?” He said: The beginning and end of the month. He said: “If you want to fast, then fast the thirteenth, fourteenth and fifteenth (of the month).`
حَدَّثَنَا أَبُو النَّضْرِ، حَدَّثَنَا الْمَسْعُودِيُّ، عَنْ حَكِيمِ بْنِ جُبَيْرٍ، عَنْ مُوسَى بْنِ طَلْحَةَ، عَنِ ابْنِ الْحَوْتَكِيَّةِ، قَالَ أُتِيَ عُمَرُ بْنُ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ بِطَعَامٍ فَدَعَا إِلَيْهِ رَجُلًا فَقَالَ إِنِّي صَائِمٌ ثُمَّ قَالَ وَأَيُّ الصِّيَامِ تَصُومُ لَوْلَا كَرَاهِيَةُ أَنْ أَزِيدَ أَوْ أَنْقُصَ لَحَدَّثْتُكُمْ بِحَدِيثِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ حِينَ جَاءَهُ الْأَعْرَابِيُّ بِالْأَرْنَبِ وَلَكِنْ أَرْسِلُوا إِلَى عَمَّارٍ فَلَمَّا جَاءَ عَمَّارٌ قَالَ أَشَاهِدٌ أَنْتَ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَوْمَ جَاءَهُ الْأَعْرَابِيُّ بِالْأَرْنَبِ قَالَ نَعَمْ فَقَالَ إِنِّي رَأَيْتُ بِهَا دَمًا فَقَالَ كُلُوهَا قَالَ إِنِّي صَائِمٌ قَالَ وَأَيُّ الصِّيَامِ تَصُومُ قَالَ أَوَّلَ الشَّهْرِ وَآخِرَهُ قَالَ إِنْ كُنْتَ صَائِمًا فَصُمْ الثَّلَاثَ عَشْرَةَ وَالْأَرْبَعَ عَشْرَةَ وَالْخَمْسَ عَشْرَةَ‏.‏