Sayyiduna Abu Umamah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) would perform ablution with half a mudd (of water).
Imam al-Tabarani recorded this report in al-Mu'jam al-Kabir; its chain contains a narrator, al-Salt bin Dinar, on whose weakness the hadith scholars are agreed.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1106