Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2190 (DarusSalam) ⓘ

It was narrated that Silah said: "We were with 'Ammar and a roast sheep was brought and he said: 'Eat.' One of the people turned away and said: 'I am fasting 'Ammar said: Whoever fasts on the day concerning which there is doubt, has disobeyed Abu Al-Qasim."' '

أَخْبَرَنَا عَبْدُ اللَّهِ بْنُ سَعِيدٍ الأَشَجُّ، عَنْ أَبِي خَالِدٍ، عَنْ عَمْرِو بْنِ قَيْسٍ، عَنْ أَبِي إِسْحَاقَ، عَنْ صِلَةَ، قَالَ كُنَّا عِنْدَ عَمَّارٍ فَأُتِيَ بِشَاةٍ مَصْلِيَّةٍ فَقَالَ كُلُوا ‏.‏ فَتَنَحَّى بَعْضُ الْقَوْمِ قَالَ إِنِّي صَائِمٌ ‏.‏ فَقَالَ عَمَّارٌ مَنْ صَامَ الْيَوْمَ الَّذِي يُشَكُّ فِيهِ فَقَدْ عَصَى أَبَا الْقَاسِمِ صلى الله عليه وسلم ‏.‏