Yahya related to me from Malik from Ibn Shihab that Sad ibn AbiWaqqas used to pray witr after isha with one raka.Maliksaid, "This is not the situation with us. Rather three is the minimumfor witr."
App reference: Book 7, Hadith 21
USC-MSA web (English) reference: Book 7, Hadith 213
Arabic reference: Book 7, Hadith 275