The Prophet ﷺ used to bathe with a saa of water and perform ablution with a mudd.
كان النبي ﷺ يغتسل بالصاع، ويتوضأ بالمد.
Abu Dawud (93) narrated it from Imam Ahmad, and it is in his Musnad (4250), and Ibn Khuzayma (117) graded it sahih, all of them via the route of Yazid bin Abu Ziyad, from Salim bin Abu Jaad, from Jabir, and he related it.Salim bin Abu Jaad is trustworthy, Ibn Main, Abu Zura, and Nasai all confirmed his reliability. But the one who narrated from him, Yazid bin Abu Ziyad, is daif, except that Ibn Adi said, "Despite his weakness, his hadith is recorded."Ibn Majah (269) also narrated it from Hisham bin Ammar, who said, Rabi bin Badr narrated to us, from Abu Zubayr, from Jabir, with the same wording. This Rabi bin Badr is extremely daif.However, it is strengthened by what Jabir said, as mentioned before, that a man said, "That is not enough for us, O Jabir!" (meaning a saa of water for ghusl), and Jabir replied, "It was enough for one who was better than you and had more hair." With this, the hadith rises to the level of hasan.