(Imām Bukhārī narrates): Abdullah bin Muhammad narrated to us, he said: Abdus Samad narrated to me, he said: Shu'bah narrated to me, he said: Abu Bakr bin Hafs narrated to me, he said: I heard Abu Salamah, he said:
I and Hazrat Aisha's (may Allāh be pleased with her) brother went to Hazrat Aisha; Hazrat Aisha's brother asked her about the ghusl of the Messenger of Allāh ﷺ. Hazrat Aisha called for a vessel of about four litres of water, then she bathed and poured water over her head, and there was a screen between us and Hazrat Aisha.
Imam Abu Abdullah (Bukhari) said: Yazid bin Harun, Bahz, and Al-Juddi have narrated from Shu'bah: approximately four litres.
Reference: Sahih Al Bukhari 251