When the Messenger of Allah ﷺ sat at the end of his Witr prayer, he would say, meaning after the salutation, "Glory be to the King, the Most Holy," three times.
كان رسول الله ﷺ إذا جلس في آخر صلاته في الوتر يقول: يعني بعد التسليم: سبحان الملك القدوس ثلاثا.
Nasai (1699) narrated it through the route of Sufyan, from Zubayd, from Saeed bin Abdul-Rahman bin Abza, from his father, from Ubayy bin Kaab, and he mentioned it. Its chain of transmission is sahih, and it was already discussed in the chapter on what is recited in Witr.