The son of Abd al-Rahman bin Abza narrates from his father, Sayyiduna Abd al-Rahman bin Abza (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) used to recite in witr "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 when he gave the salam he would say: 'Subhan al-Malik al-Quddus' (Glory be to the King, the Most Holy) three times, raising his voice the third time.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4869