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 319 b

'A'isha reported:

The Messenger of Allah (ﷺ) took a bath from the vessel (which contained seven to eight seers, i. e. fifteen to sixteen pounds) of water And I and he (the Holy Prophet) took a bath from the same vessel. And in the hadith narrated by Sufyan the words are:" from one vessel". Qutaiba said: Al-Faraq is three Sa' (a cubic measuring of varying magnitude).
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا لَيْثٌ، ح وَحَدَّثَنَا ابْنُ رُمْحٍ، أَخْبَرَنَا اللَّيْثُ، ح وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ وَعَمْرٌو النَّاقِدُ وَزُهَيْرُ بْنُ حَرْبٍ قَالُوا حَدَّثَنَا سُفْيَانُ، كِلاَهُمَا عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَغْتَسِلُ فِي الْقَدَحِ وَهُوَ الْفَرَقُ وَكُنْتُ أَغْتَسِلُ أَنَا وَهُوَ فِي الإِنَاءِ الْوَاحِدِ ‏.‏ وَفِي حَدِيثِ سُفْيَانَ مِنْ إِنَاءٍ وَاحِدٍ ‏.‏ قَالَ قُتَيْبَةُ قَالَ سُفْيَانُ وَالْفَرَقُ ثَلاَثَةُ آصُعٍ ‏.‏
Reference : Sahih Muslim 319b
In-book reference : Book 3, Hadith 49
USC-MSA web (English) reference : Book 3, Hadith 625
  (deprecated numbering scheme)