Al-Kāfi - Volume 3 > Necessity of Ghusl (bath) on Friday
Hadith #4

4ـ عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ عَلِيِّ بْنِ سَيْفٍ عَنْ أَبِيهِ سَيْفِ بْنِ عَمِيرَةَ عَنِ الْحُسَيْنِ بْنِ خَالِدٍ قَالَ سَأَلْتُ أَبَا الْحَسَنِ الأوَّلَ (عَلَيْهِ الْسَّلام) كَيْفَ صَارَ غُسْلُ يَوْمِ الْجُمُعَةِ وَاجِباً فَقَالَ إِنَّ الله تَبَارَكَ وَتَعَالَى أَتَمَّ صَلاَةَ الْفَرِيضَةِ بِصَلاَةِ النَّافِلَةِ وَأَتَمَّ صِيَامَ الْفَرِيضَةِ بِصِيَامِ النَّافِلَةِ وَأَتَمَّ وُضُوءَ الْفَرِيضَةِ بِغُسْلِ يَوْمِ الْجُمُعَةِ مَا كَانَ فِي ذَلِكَ مِنْ سَهْوٍ أَوْ تَقْصِيرٍ أَوْ نِسْيَانٍ أَوْ نُقْصَانٍ.

4. A number of our people have narrated from Ahmad ibn Muhammad from Ali ibn Sayf from his father Sayf ibn ‘Amirah from al- Husayn ibn Khalid who has said the following: “I asked abu al-Hassan, al-Awwal, ‘Alayhi al-Salam, how did Ghusl (bath) for Friday become obligatory? He (the Imam) said, ‘Allah, most Blessed, most High, has completed obligatory Salah (prayer) with optional ones. He has completed obligatory fasting with optional fast. He has completed obligatory Wudu’ with Ghusl (bath) on Friday in case a mistake, shortcoming, forgetfulness or flaws may take place (in such Wudu’).”’