Short answer
Surah al-Kahf. Reading it on Friday is mustahabb (recommended) in the Hanafi school and the other schools, based on hadith that it gives you light from one Friday to the next. Its time runs from sunset on Thursday to sunset on Friday. Sending abundant salawat on the Prophet ﷺ is the other key Friday practice.
Reciting Surah al-Kahf on Friday, in the day or the night before it, is mustahabb. In the Fajr prayer of Friday the Prophet ﷺ often recited Surah al-Sajdah and Surah al-Insan; the Hanafis say an imam should do so sometimes but not always, so people do not think it is obligatory.
The four schools agree on this.
Explanation
Surah al-Kahf. The hadith that whoever reads Surah al-Kahf on Friday will have a light between the two Fridays is reported by al-Hakim, al-Bayhaqi and others, and many hadith scholars graded it authentic or sound. It is not in Bukhari or Muslim. In Sahih Muslim there is the related virtue that whoever memorises the first ten ayat of al-Kahf is protected from the Dajjal. The four schools agree that reading it on Friday is recommended.
When: the Islamic day starts at sunset, so the night of Friday (Thursday evening) counts, and the time ends at Maghrib on Friday. You can read it in one sitting or spread it through the day, from a mushaf, from memory or on your phone. Reading it in a group is fine; listening to it is good but is not the same as reading: see listening versus reading.
Other Friday recitation:
- In the Fajr prayer, the Prophet ﷺ used to recite Surah al-Sajdah (32) in the first rakah and Surah al-Insan (76) in the second.
- In the Jumuah prayer he recited al-A'la and al-Ghashiyah, or al-Jumuah and al-Munafiqun.
- Sending many salawat on him, since he said they are presented to him (Abu Dawud).
For the full list of Friday sunnahs, see sunnahs of Friday and Surah al-Kahf.
The evidence
Quran, Surah Al-Jumu'a 62:9
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِذَا نُودِىَ لِلصَّلَوٰةِ مِن يَوْمِ ٱلْجُمُعَةِ فَٱسْعَوْا۟ إِلَىٰ ذِكْرِ ٱللَّهِ وَذَرُوا۟ ٱلْبَيْعَ ۚ ذَٰلِكُمْ خَيْرٌۭ لَّكُمْ إِن كُنتُمْ تَعْلَمُونَ ﴿٩﴾
O you who have believed, when [the adhan] is called for the prayer on the day of Jumu'ah [Friday], then proceed to the remembrance of Allah and leave trade. That is better for you, if you only knew.
Friday is a day set apart for the remembrance of Allah.
Sahih Muslim 809
Sahih... من حفظ عشر آيات من أول سورة الكهف عصم من الدجال ...
"Whoever memorises ten ayat from the beginning of Surah al-Kahf will be protected from the Dajjal."
An authentic virtue of Surah al-Kahf.
Sahih al-Bukhari 891
Sahih... يقرأ في الجمعة في صلاة الفجر الم تنزيل السجدة ...
"On Friday, in the Fajr prayer, the Prophet used to recite Alif Lam Mim Tanzil (al-Sajdah) and Hal ata ala al-insan."
The sunnah recitation in Fajr on Friday.
Sunan Abi Dawud 1047
Grading to be checked... فأكثروا علي من الصلاة فيه فإن صلاتكم معروضة علي ...
"So send many blessings on me on that day, for your blessings are presented to me."
Abundant salawat is a Friday sunnah.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, chapter on Jumuah. Lists reading Surah al-Kahf among the recommended acts of Friday and its night.
- al-Durr al-Mukhtar, Kitab al-Salah, chapter on the description of prayer, on fixing a surah. Disliked to always fix a particular surah for a prayer.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.