كتاب الصيام
13
The Book of Fasting
(24)
Chapter: It is disliked to single out Friday for fasting, unless it coincides with a day that one customarily fasts
(24)
باب كَرَاهِيَةِ صِيَامِ يَوْمِ الْجُمُعَةِ مُنْفَرِدًا

Abu Huraira (Allah be pleased with him) reported Allah's Apostle (ﷺ) as saying:

Do not single out the night (preceding) Friday among the nights for prayer and do not single out Friday among days for fasting but only when anyone among you is accustomed to fast (on dates) which coincide with this day (Friday).
وَحَدَّثَنِي أَبُو كُرَيْبٍ، حَدَّثَنَا حُسَيْنٌ، - يَعْنِي الْجُعْفِيَّ - عَنْ زَائِدَةَ، عَنْ هِشَامٍ، عَنِ ابْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ، - رضى الله عنه - عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ لاَ تَخْتَصُّوا لَيْلَةَ الْجُمُعَةِ بِقِيَامٍ مِنْ بَيْنِ اللَّيَالِي وَلاَ تَخُصُّوا يَوْمَ الْجُمُعَةِ بِصِيَامٍ مِنْ بَيْنِ الأَيَّامِ إِلاَّ أَنْ يَكُونَ فِي صَوْمٍ يَصُومُهُ أَحَدُكُمْ ‏"‏ ‏.‏
Reference : Sahih Muslim 1144 b
In-book reference : Book 13, Hadith 190
USC-MSA web (English) reference : Book 6, Hadith 2546
  (deprecated numbering scheme)