Hadithcore
Riyad as-Salihin, 1227

'Ammar bin Yasir (May Allah be pleased with them) reported:

He who observes the fast on a doubtful day, has in fact disobeyed Abul-Qasim, (i.e., Messenger of Allah (ﷺ)). <b>[Abu Dawud and At- Tirmidhi]</b>.

وعن أبي اليقظان عمار بن ياسر، رضي الله عنهما قال‏:‏ ‏

‏من صام اليوم الذي يشك فيه فقد عصي أبا القاسم، صلى الله عليه وسلم‏

‏ ‏(‏‏(‏رواه أبو داود والترمذي وقال‏:‏ حديث حسن صحيح‏)‏‏)‏‏.‏

References1 variant
In-Book Reference
Book 8, Hadith 237
Sharh · explanationclick to expand
<b>Guidance from the Hadith:</b><br> 1) It prohibits the fasting of the day of doubt, which precedes the confirmed start of Ramadan, based on prohibition reported from the Prophet (may Allah’s peace and blessings be upon him).<br> 2) Violation of the Sunnah and the prohibitions laid down by the Prophet leads to misguidance and disunity, even if the person thinks he is doing good. “How often a person intends good, yet he does not attain it!”<br> <br> <b>Benefit:</b><br> Fasting before Ramadan is of three types:<br> First: from mid-Sha‘bān to the 28th of it. This fasting is disliked, unless a person has a certain habit of fasting.<br> Second: one or two days before Ramadan. This fasting is prohibited, unless a person has a certain habit of fasting.<br> Third: Fasting on the day of doubt. One may not fast on it even on a voluntary basis. This is because worship should not be done except on the basis of certainty. Moreover, fasting on this day might confuse people and make them think this is Ramadan.