Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.122A.

‘Ammar ibn Yasir said: “Whoever fasts the ‘day of doubt has disobeyed Abu alQasim [the Prophet].” This is related by an-Nasa’i, at-Tirmizhi, Abu Dawud, and Ibn Majah.

Of its status, at-Tirmizhi says: “It is a hassan sahih hadith. Most of the knowledgeable people act in accordance with it. It is the opinion of Sufyan ath-Thauri, Malik ibn Anas, ‘Abdullah ibn alMubarak, ash-Shaf’i, Ahmad, and Ishaq. They all hate that one fasts on a ‘day of doubt.’ Most of them believe that if one fasts on such a day and it turns out to be Ramadan, then that day still has to be made up later. If such a day occurs during one’s regular fasting period, then it is permissible to fast on such a day.”

As related by “the group,” Abu Hurairah reported that the Messenger of Allah said: “Do not precede Ramadan by fasting the day or two before it unless it is a day on which the person usually fasts.”

About this hadith, at-Tirmizhi says: “The hadith is hassan sahih and the scholars act in accordance with it. They dislike that a person should hasten Ramadan by fasting on the day before it. If a person usually fasts on a day and ‘the day of doubt’ occurs on that day, then there is no problem with his fasting on that day, in their opinion.”