The Messenger of Allah – sallallaahu alaihi wa sallam – when he would give salaams after the witr prayer would say :
سُبحانَ الملِكِ القدُّوس

Three times and the third time he would raise his voice. (1)

Classified as Saheeh by Imam Al-Albaani in Saheeh an-Nisaa’i Hadeeth number 1732

(1) And in Hadeeth 1733 – also classified as Saheeh by Imam Al-Albaani – he would elongate the third time.