Musnad Ahmad 1107:0

Narrated Abu Basrah al-Ghifari: The Messenger of Allah (ﷺ) said: "Indeed, Allah has added for you a prayer, and it is the Witr. So perform it between the Isha prayer and the dawn." He said: "And he (the Prophet) used to say in his Witr: 'Glory be to the King, the Holy' (Subbuhun Quddusun), three times, and he would extend his voice in the third time."