Al-Kāfi - Volume 3 > The Amount of Water Needed for Ghusl (bath) of a Deceased Person
Hadith #3

3ـ مُحَمَّدُ بْنُ يَحْيَى قَالَ كَتَبَ مُحَمَّدُ بْنُ الْحَسَنِ إِلَى أَبِي مُحَمَّدٍ (عَلَيْهِ الْسَّلام) فِي الْمَاءِ الَّذِي يُغَسَّلُ بِهِ الْمَيِّتُ كَمْ حَدُّهُ فَوَقَّعَ (عَلَيْهِ الْسَّلام) حَدُّ غُسْلِ الْمَيِّتِ يُغْسَلُ حَتَّى يَطْهُرَ إِنْ شَاءَ الله قَالَ وَكَتَبَ إِلَيْهِ هَلْ يَجُوزُ أَنْ يُغَسَّلَ الْمَيِّتُ وَمَاؤُهُ الَّذِي يُصَبُّ عَلَيْهِ يَدْخُلُ إِلَى بِئْرِ كَنِيفٍ أَوِ الرَّجُلُ يَتَوَضَّأُ وُضُوءَ الصَّلاَةِ أَنْ يُصَبَّ مَاءُ وُضُوئِهِ فِي كَنِيفٍ فَوَقَّعَ (عَلَيْهِ الْسَّلام) يَكُونُ ذَلِكَ فِي بَلاَلِيعَ.

3. Muhammad ibn Yahya has said that Muhammad ibn al-Hassan who has narrated the following: “I once wrote to abu Muhammad, ‘Alayhi al-Salam, ‘How much water is needed for Ghusl (bath) of a deceased person?’ The answer came with the signature of the Imam, ‘The amount of water needed is to wash the dead body clean, by the will Allah.’ He (the narrator) has said that he wrote again to him (the Imam) asking, ‘Is it permissible to wash a deceased person where the water poured on him flows into the toilet or cesspool or if water with which one takes Wudu’ flows in the toilet?’ He (the Imam) wrote back to him with his signature, ‘It does end up in the toilet.’”