سنن النسائي

It was narrated from Sa'eed bin Abdur-Rahman bin Abza that his father said: "The Messenger of Allah (ﷺ) used to recite in witr: Glorify the Name of Your Lord, the Most High;' and "Say: O you disbelievers!'; and 'Say: He is Allah, (the) One." And after he had said the salam, he would say: Subhanal-Malikil-Quddus (Glory be to the Sovereign, the Most Holy) three times, elongating the words the third time, then raising it."