مُوَطَّأُ مَالِكٍ
Muwatta Malik 275
Tahajjud
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّ سَعْدَ بْنَ أَبِي وَقَّاصٍ، كَانَ يُوتِرُ بَعْدَ الْعَتَمَةِ بِوَاحِدَةٍ . قَالَ مَالِكٌ وَلَيْسَ عَلَى هَذَا الْعَمَلُ عِنْدَنَا وَلَكِنْ أَدْنَى الْوِتْرِ ثَلاَثٌ .
Yahya related to me from Malik from Ibn Shihab that Sad ibn Abi Waqqas used to pray witr after isha with one raka. Malik said, "This is not the situation with us. Rather three is the minimum for witr."
Sunnah.com reference: Book 7, Hadith 21