Ubayy bin Ka'b (may Allah be pleased with him) said that when the Messenger of Allah (peace and blessings be upon him) gave the salam at the end of witr, he would say: 'Subhan al-Malik al-Quddus' (Glory be to the King, the Most Holy).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4870