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

Abu Salamab. 'Abd al-Rahman reported:

I along with the foster brother of 'A'isha went to her and he asked about the bath of the Apostle (ﷺ) because of sexual intercourse. She called for a vessel equal to a Sa' and she took a bath. and there was a curtain between us and her. She poured water on her head thrice and he (Abu Salama) said: The wives of the Apostle (ﷺ) collectedhair on their heads and these lopped up to ears (and did rot go beyond that).
وَحَدَّثَنِي عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ الْعَنْبَرِيُّ، قَالَ حَدَّثَنَا أَبِي قَالَ، حَدَّثَنَا شُعْبَةُ، عَنْ أَبِي بَكْرِ بْنِ حَفْصٍ، عَنْ أَبِي سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ، قَالَ دَخَلْتُ عَلَى عَائِشَةَ أَنَا وَأَخُوهَا، مِنَ الرَّضَاعَةِ فَسَأَلَهَا عَنْ غُسْلِ النَّبِيِّ، صلى الله عليه وسلم مِنَ الْجَنَابَةِ فَدَعَتْ بِإِنَاءٍ قَدْرِ الصَّاعِ فَاغْتَسَلَتْ وَبَيْنَنَا وَبَيْنَهَا سِتْرٌ وَأَفْرَغَتْ عَلَى رَأْسِهَا ثَلاَثًا ‏.‏ قَالَ وَكَانَ أَزْوَاجُ النَّبِيِّ صلى الله عليه وسلم يَأْخُذْنَ مِنْ رُءُوسِهِنَّ حَتَّى تَكُونَ كَالْوَفْرَةِ ‏.‏
Reference : Sahih Muslim 320
In-book reference : Book 3, Hadith 50
USC-MSA web (English) reference : Book 3, Hadith 626
  (deprecated numbering scheme)