It was narrated from Abu Bakr bin Hafs:

"I heard Abu Salamah say: 'I entered upon 'Aishah and her foster-brother was with her. He asked her about the Ghusl of the Prophet (ﷺ). She called for a vessel in which was a Sa' of water, then she concealed herself and performed Ghusl and poured water over her head three times.'"

أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ أَبِي بَكْرِ بْنِ حَفْصٍ، سَمِعْتُ أَبَا سَلَمَةَ، يَقُولُ دَخَلْتُ عَلَى عَائِشَةَ - رضى الله عنها - وَأَخُوهَا مِنَ الرَّضَاعَةِ فَسَأَلَهَا عَنْ غُسْلِ النَّبِيِّ صلى الله عليه وسلم فَدَعَتْ بِإِنَاءٍ فِيهِ مَاءٌ قَدْرَ صَاعٍ وَسَتَرَتْ سِتْرًا فَاغْتَسَلَتْ فَأَفْرَغَتْ عَلَى رَأْسِهَا ثَلاَثًا ‏.‏