Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) used to pray three rak'ahs of witr, reciting in them "Glorify the name of your Lord, the Most High" [Surah al-A'la: 1], "Say: O disbelievers" [Surah al-Kafirun: 1] and "Say: He is Allah, the One" [Surah al-Ikhlas: 1], and he would recite the qunut supplication before the bowing. When he gave the salam, he would say 'Subhan al-Malik al-Quddus' (Glory be to the King, the Most Holy) twice quietly, and the third time aloud, raising his voice.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4861