The Amount of Water for Ablution and Ghusl
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 8
وَقَالَ أَبُو جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ: «اِغْتَسَلَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ هُوَ وَ زَوْجَتُهُ مِنْ خَمْسَةِ أَمْدَادٍ مِنْ إِنَاءٍ وَاحِدٍ» فَقَالَ لَهُ زُرَارَةُ كَيْفَ صَنَعَ فَقَالَ «بَدَأَ هُوَ فَضَرَبَ يَدَهُ فِي اَلْمَاءِ قَبْلَهَا فَأَنْقَى فَرْجَهُ ثُمَّ ضَرَبَتْ هِيَ فَأَنْقَتْ فَرْجَهَا ثُمَّ أَفَاضَ هُوَ وَ أَفَاضَتْ هِيَ عَلَى نَفْسِهَا حَتَّى فَرَغَا وَ كَانَ اَلَّذِي اِغْتَسَلَ بِهِ اَلنَّبِيُّ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ ثَلاَثَةَ أَمْدَادٍ وَ اَلَّذِي اِغْتَسَلَتْ بِهِ مُدَّيْنِ وَ إِنَّمَا أَجْزَأَ عَنْهُمَا لِأَنَّهُمَا اِشْتَرَكَا فِيهِ جَمِيعاً وَ مَنِ اِنْفَرَدَ بِالْغُسْلِ وَحْدَهُ فَلاَ بُدَّ لَهُ مِنْ صَاعٍ..
Abu Ja'far Imam Muhammad ibn Ali Al-Baqir عليه السلام said: "The Messenger of Allah صلى الله عليه وعلى آله وسلم and his wife performed ghusl using five amdad from one container." Zurarah then asked how it was done. Imam عليه السلام said: "He started by immersing his hand in the water before hers and cleansed his private part. Then She did the same and cleansed her private part. After that, he poured water over himself, and She poured water over herself until they were finished. The amount that the Prophet used for ghusl was three amdad, while She used two muds. This was sufficient for both of them because they shared the same water. However, for someone who performs ghusl alone, they must use a saa'."
- Ṣaḥīḥ al-Faqīh