Did the Messenger (may Allah's peace and blessings be upon him) prohibit the fasting of Friday? He said: Yes
Authentic hadithعن 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».
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!"
Explanation
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.
Source · An-Nasaa’i - Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).