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.'"
English reference: Vol. 1, Book 1, Hadith 228
Arabic reference: Book 1, Hadith 229