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بسم الله الرحمن الرحيم

As-Samma’, the sister of Abdullāh ibn Busr narrated:

The Prophet (ﷺ) said: ‘Do not fast on Saturday except what has been made obligatory on you; and if one of you can get nothing but a grape skin or a piece of wood from a tree, he should chew it.’ (Sunan Abī Dawūd 2421 and others. Declared Sahīh upon the conditions of Al-Bukhārī by al-Hākim (in al-Mustadrak 2/343) and Al-Albānī (Irwā 4/118). Also authenticated by Ibn Taymiyyah in Sharh al-‘Umdah, Ibn Qudāmah in al-Kāfī, Ibn Hajr in Hidāyah Rawāh, and Al-Arnāūt in Sharh as-Sunnah 3/465)

Al-Tahāwī says: “A group of people (of knowledge) inclined to this hadīth and disliked the voluntary fast on Saturday.”

Al-‘Aynī said in explanation: “The isnād is Sahīh [he then explains the men of the isnād]…

A group of people, meaning Mujāhid [student of Ibn’ Abbās], Tawūs ibn Kaysān, Ibrāhīm (al-Nakhaī), Khālid ibn Ma’dān.”

(Sharh Ma’ānī al-Āthār of Imām al-Tahāwī, Book of Fasting, pg. 433)

A summary of the points made by Shaykh Al-Albānī regarding the fasting on Saturday:

• the Hadīth of Juwayrīyah (“…did you fast a day before it…will you fast a day after it, she said no…”) permits the fasting of Saturday, and the other Hadīth prohibits it, so the prohibition takes precedence over the permit.
• The fasting on Saturday is not permitted except for an obligation as was stated [by the Messenger  (ﷺ) : except what has been made obligatory on you] , there is no difference between fasting it on its own and combining a day before it or a day after it with it.
• As for those who permit the fasting on Saturday and state that the statement; “…except what has been made obligatory on you..” that this is only when it is singled out, what are they using as evidence here, a principle, a text? (Rather) this text prohibits all types of fasting [on Saturday] except those that have been obligated.
• [TN: this is similar to what Ibn al-Qayyim said that the prohibition here includes all types of fast, whether done alone or coupled with another day. The exception is only the obligatory fast, otherwise if the prohibition was fasting the day alone the Prophet (ﷺ) would have said “do not fast on Saturday unless you fast the day before it or after it”, as he (ﷺ) said regarding the fasting of Friday. (Sharh Sunan Abū Dawūd 4/666)]
• The narrator of the Hadīth said: Whoever fasts on Saturday has neither fasted nor has he eaten.

(Summarised from https://subulassalaam.com/articles/article.cfm?article_id=92#1)