عَنْ صِلَةَ بْنِ زُفَرَ، قَالَ 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 صلى الله عليه وسلم.

Silah bin Zufar said:

“We were with Ammar bin 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 Abul-Qasim (ﷺ).”

Reported by Abu Dawud and others. Graded saheeh by Shaykh Albani rahimahullah

Note: Therefore, it is not permitted to fast on the day of doubt [i.e. 30th day of Sha’ban] unless it were to coincide with a day of fast which the person usually fasts on or a day which the person uses to make up for what was upon him of fast from last Ramadan and this is by the consensus of the scholars.

Mentioned by

AbdulFattaah Bin Uthman
Abu Fajr