Al-Auzai said: I always concealed it, but I found that it became known widely, that is, the tradition on Ibn Busr about fasting on Saturday. Abu Dawud said: Malik said: This is a false (tradition).
Explanation & Benefits
Shaykh Umar Farooq Saeedi
Benefits and Issues:
From the aforementioned details, it is clear that just as it is prohibited to fast solely on Friday, it is likewise prohibited to fast solely on Saturday.
However, if one combines the fast of Saturday with that of Sunday or Friday, then fasting on Friday and Saturday becomes permissible.
Similarly, on these two days (Friday and Saturday), it is permissible to observe an obligatory fast, a fast of vow (nadhr), a make-up fast for missed obligatory fasts (qada), an expiatory fast (kaffarah), or if the day of Arafah or Ashura falls on these days, then fasting on those days is also permissible. All such fasts are allowed to be observed.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2424