´It was narrated from ‘Abdullah bin Busr that the Messenger of Allah (ﷺ) said:` “Do not fast on Saturdays apart from days when you are obliged to fast. If anyone of you cannot find anything other than grape stalks or the bark of a tree, let him suck on it.” Another chain from 'Abdullah bin Busr, from his sister who said: "The Messenger of Allah (ﷺ) said," and he mentioned similarly.
Explanation & Benefits
Maulana Ataullah Sajid
Benefits and Issues:
➊ This also establishes the prohibition of fasting on the day of Saturday alone.
When observing obligatory fasts, this day also comes, but that is not a fast of Saturday alone. Similarly, while making up missed fasts (qada), there is no need to be particular about not fasting on Saturday. Likewise, if by coincidence a fast falls on a Saturday—for example, if someone had one fast left to make up and he observed it, and it happened to be a Saturday—then the intention of the one fasting was not to give special importance to Saturday. Or, while observing the fast of Dawud (alayhis salam), if he fasted on Thursday, then now he would have to fast again on Saturday and then on Monday. In such cases, it will not be prohibited.
Source: Commentary on Sunan Ibn Mājah by Mawlānā ‘Atā’ullāh Sājid, Page: 1726