Al-Kāfi - Volume 3 > Sprucing up on Fridays
Hadith #9

9ـ حَمَّادٌ عَنْ حَرِيزٍ عَنْ بَعْضِ أَصْحَابِنَا عَنْ أَبِي جَعْفَرٍ (عَلَيْهِ الْسَّلام) قَالَ لاَ بُدَّ مِنْ غُسْلِ يَوْمِ الْجُمُعَةِ فِي الْحَضَرِ وَالسَّفَرِ فَمَنْ نَسِيَ فَلْيُعِدْ مِنَ الْغَدِ. وَرُوِيَ فِيهِ رُخْصَةٌ لِلْعَلِيلِ.

9. Flammad has narrated from Flariz from certain persons of our people who has narrated the following: Abu Ja’far, ‘Alayhi al-Salam, has said, ‘Taking Ghusl (bath) on Friday is necessary, regardless of whether one is at home or on a journey. If one forgets, one must take Ghusl (bath) the next day. In the case of a person suffering from an illness it is permissible not to take a Ghusl (bath).”’ (Ahadith 8-9 are best explained in the form of fatwa.)