Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "One mudd (of water) suffices for ablution, and one sa' (of water) for the ritual bath of major ritual impurity."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 935