كتاب الحيض
3  
The Book of Menstruation
(10)
Chapter: The amount of water with which it is recommended to perform ghusl in the case of janabah; a man and woman washing from a single vessel; one of them washing with the left-over water of the other
(10)
باب الْقَدْرِ الْمُسْتَحَبِّ مِنَ الْمَاءِ فِي غُسْلِ الْجَنَابَةِ وَغُسْلِ الرَّجُلِ وَالْمَرْأَةِ فِي إِنَاءٍ وَاحِدٍ فِي حَالَةٍ وَاحِدَةٍ وَغُسْلِ أَحَدِهِمَا بِفَضْلِ الآخَرِ
Sahih Muslim 326 a

Safina reported:

The Messenger of Allah (ﷺ) took a bath with one Sa` of water because of sexual intercourse and performed ablution with one Mudd.
وَحَدَّثَنَا أَبُو كَامِلٍ الْجَحْدَرِيُّ، وَعَمْرُو بْنُ عَلِيٍّ، كِلاَهُمَا عَنْ بِشْرِ بْنِ الْمُفَضَّلِ، - قَالَ أَبُو كَامِلٍ حَدَّثَنَا بِشْرٌ، - حَدَّثَنَا أَبُو رَيْحَانَةَ، عَنْ سَفِينَةَ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُغَسِّلُهُ الصَّاعُ مِنَ الْمَاءِ مِنَ الْجَنَابَةِ وَيُوَضِّؤُهُ الْمُدُّ ‏.‏
Reference : Sahih Muslim 326a
In-book reference : Book 3, Hadith 60
USC-MSA web (English) reference : Book 3, Hadith 636
  (deprecated numbering scheme)