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

'A'isha reported:

I and the Messenger of Allah (ﷺ) took a bath from one vessel which was placed between me and him and he would get ahead of me, so that I would say: Spare (some water for) me, spare (some water for) me; and she said that they had had sexual intercourse.
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا أَبُو خَيْثَمَةَ، عَنْ عَاصِمٍ الأَحْوَلِ، عَنْ مُعَاذَةَ، عَنْ عَائِشَةَ، قَالَتْ كُنْتُ أَغْتَسِلُ أَنَا وَرَسُولُ اللَّهِ، صلى الله عليه وسلم مِنْ إِنَاءٍ - بَيْنِي وَبَيْنَهُ - وَاحِدٍ فَيُبَادِرُنِي حَتَّى أَقُولَ دَعْ لِي دَعْ لِي ‏.‏ قَالَتْ وَهُمَا جُنُبَانِ ‏.‏
Reference : Sahih Muslim 321 d
In-book reference : Book 3, Hadith 54
USC-MSA web (English) reference : Book 3, Hadith 630
  (deprecated numbering scheme)