Whoever fasts on the day about which people are in doubt has disobeyed Abu Qasim ﷺ.
حدثنا أبو سعيد عبد الله بن سعيد الأشج حدثنا أبو خالد الأحمر عن عمرو بن قيس الملائي عن أبي إسحق عن صلة بن زفر قال كنا عند عمار بن ياسر فأتي بشاة مصلية فقال كلوا فتنحى بعض القوم فقال إني صائم
من صام اليوم الذي يشك فيه الناس فقد عصى أبا القاسم ﷺ
[Its chain is daif]. Abu Dawud recorded it in the Book of Fasting, chapter on the dislike of fasting on the day of, no. 2334, and Ibn Majah, no. 1645, from the hadith of Abu Khalid Ahmar, with this wording. Nasai narrated it, 4/153, no. 2190, from Abu Saeed Ashaj, with this wording. Bukhari cited it without a full chain, no. 1906, and Ibn Khuzayma graded it sahih, no. 1914, as did Ibn Hibban, no. 878, and Daraqutni, 2/157, and Hafiz in Taghliq Taliq, 2/41, and Hakim said it meets the standard of the two Shaykhs, 1/424, and Dhahabi agreed with him.And on this subject there is also a report from Abu Hurairah [Bazzar (Kashf), 1/498, no. 1066, and its chain is very daif] and from Anas [Bayhaqi, 4/209, and it is mawquf, and in it is Ahmad bin Hasan bin Yazid Ibn Majah Qazwini, and I did not find anyone who declared him trustworthy].