Hadith: Did the Messenger (may Allah's peace and blessings be upon him) prohibit the fasting of Friday? He said: Yes

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».
[صحيح] - [رواه البخاري ومسلم -ولفظ مسلم: (نعم وَرَبِّ هذا البيت) أما لفظ : "ورب الكعبة" فهذا لفظ النسائي في الكبرى، نبه على ذلك الشيخ ابن عثيمين -رحمه الله]

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!"
[Authentic hadith] - [An-Nasaa’i - Narrated by Bukhari & Muslim]

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.