Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.017.

It is preferred for a person to say after the taslim: “Glory be to the Master, the Holy,” three times aloud, saying the third time: “Lord of the angels and the souls.” Abu Dawud and anNasa’i record that Ubayy ibn Ka’b said: “The Prophet sallallahu alehi wasallam would recite al-A’la and al-Kafirun in the witr prayer. When he made the taslim, he would say: ‘Glory be to the Master, the Holy,’ three times, prolonging the third repetition and saying it aloud.” This is the wording in which an-Nasa’i recorded it. Ad-Daraqutni has the addition: “And he would say, ‘Lord of the angels and the spirits. “‘  He would then make supplications and, according to what Ahmad, anNasa’i, Abu Dawud, Ibn Majah, and at-Tirmizhi record from ‘Ali, he would say at the end of his witr: “O Allah, I seek refuge in Your pleasure from your anger. And I seek refuge in Your granting well being from Your punishment. And I seek refuge in You from You. I cannot reckon Your praise: You are as You have praised Yourself.”