Answered by Shaykh Abdurrahman ibn Yusuf Mangera
Is there a hadith that prohibits fasting on Saturday?
Assalamu alaykum
In the name of Allah the Inspirer of truth.
The Messenger of Allah (upon him be peace) said, “Whoever emulates a people is from them.”
He also told people to fast an additional day with the fast of Ashura when someone told him that the Jews fasted on that day too.
This is so that our practices remain distinct from that of other religions. Hence, the jurists have considered it disliked to fast on Saturday alone without fasting another day with it (Friday or Sunday). Hence, there does not seem to be any specific hadith regarding this prohibition but the general hadith on the prohibition of emulating practices of other religions applies here too. However, if a person fasts according to another schedule or routine he has, for instance, every second day, and that fast happens to fall on a Saturday then the prohibition would not apply (Radd al-Muhtar 2:84)
And Allah knows best.
Wassalam
Abdurrahman ibn Yusuf