Musannaf Ibn Abi Shayba 1:372

Ibn ‘Ulayyah narrated to us, from Hisham al-Dastawa’i, from Yahya ibn Abi Kathir, who said: Abu Salamah narrated to us, from Zaynab bint Umm Salamah, from Umm Salamah, that she and the Messenger of Allah (may Allah bless him and grant him peace) used to perform ghusl (ritual bathing) from a single vessel.