Prohibition to eat on the day of doubt (whether the crescent moon could be sighted)

We were with Ammar b. Yasir when a roasted sheep was brought and he said: "Eat." Someone among the people said: "I am fasting." So Ammar said: "Whoever fasts on a day in which there is doubt, then he has disobeyed Abu al-Qasim ﷺ."  

كُنَّا عِنْدَ عَمَّارِ بْنِ يَاسِرٍ فَأُتِيَ بِشَاةٍ مَصْلِيَّةٍ فَقَالَ كُلُوا فَتَنَحَّى بَعْضُ الْقَوْمِ فَقَالَ إِنِّي صَائِمٌ فَقَالَ عَمَّارٌ مَنْ صَامَ الْيَوْمَ الَّذِي يَشُكُّ فِيهِ النَّاسُ فَقَدْ عَصَى أَبَا الْقَاسِمِ ﷺ