ما يحرم على الصائم
Muhammad ibn ‘Abbād ibn Ja‘far reported: I asked Jābir ibn ‘Abdullāh: "Did the Messenger (may Allah's peace and blessings be upon him) prohibit the fasting of Friday?" He said: 'Yes.' In another version: "By the Lord of the Ka‘bah!"
عن محمد بن عباد بن جعفر قال: «سألت جابر بن عبد الله -رضي الله عنهما-: أَنَهَى النبي -صلى الله عليه وسلم- عن صوم يوم الجمعة ؟ قال: نعم». وفي رواية: «وَرَبِّ الْكَعْبَة».
شرح الحديث :
As Friday is a day of Eid for Muslims, Allah forbade that we should specifically fast it or perform Qiyām in it, unless we fast the day before or after it, or it coincides with days one usually fasts. Also, this may otherwise lead people to think that it is an obligation to perform more worship on Friday.