Her mother Umm Salama told her that she and the Messenger of Allah ﷺ used to perform the ritual bath for major impurity from a single vessel.
إن أمها أم سلمة حدثتها قالت: كانت هي ورسول الله ﷺ يغتسلان في الإناء الواحد من الجنابة.
Bukhari narrated it in the Book of Menstruation (322), and Muslim in the Book of Menstruation (324), both through Abu Salama bin Abdul-Rahman, from Zaynab the daughter of Abu Salama, from her mother, who related the hadith. It contains a story, see the Book of Menstruation, the chapter on lying with a menstruating woman under one blanket.