The Amount of Water for Ablution and Ghusl
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 8
وَقَالَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ: «اَلْوُضُوءُ مُدٌّ وَ اَلْغُسْلُ صَاعٌ وَ سَيَأْتِي أَقْوَامٌ بَعْدِي يَسْتَقِلُّونَ ذَلِكَ فَأُولَئِكَ عَلَى خِلاَفِ سُنَّتِي وَ اَلثَّابِتُ عَلَى سُنَّتِي مَعِي فِي حَظِيرَةِ اَلْقُدْسِ.
The Messenger of Allah صلى الله عليه وعلى آله وسلم said: "Ablution (wudu) is a mud (a specific measure), and ghusl (ritual washing) is a saa' (a larger measure). There will come people after me who will underestimate these amounts, and those will be contrary to my Sunnah. The one who adheres to my Sunnah will be with me in the enclosure of holiness (Paradise)."
- Ṣaḥīḥ al-Faqīh