From Sahih Muslim (Hadith 727):
A'isha reported:The Messenger of Allah (ﷺ) took a bath from the vessel (which contained seven to eight seers, i. e. fifteen to sixteen pounds) of water And I and he (the Holy Prophet) took a bath from the same vessel. And in the hadith narrated by Sufyan the words are:" from one vessel". Qutaiba said: Al-Faraq is three Sa' (a cubic measuring of varying magnitude)
Reference: Muslim 727