Musnad Ahmad 1034:170

'Abd al-Rahman ibn Mahdi narrated to us, Zaidah narrated to us, from 'Abd al-Malik ibn 'Umayr, the son of Hudhayfah's brother narrated to me, from Hudhayfah, who said: I came to the Messenger of Allah (peace and blessings of Allah be upon him) one night to pray with his prayer. He began and recited a recitation that was neither low nor high, a beautiful recitation in which he chanted, making us hear it. He said: Then he bowed for about the length of his standing, then he raised his head for about the length of his bowing, and said: "Allah hears those who praise Him." Then he said: "Praise be to Allah, the Possessor of Might, the Kingdom, the Grandeur, and the Majesty," until he finished the long [supplication], while there was darkness of the night upon him. 'Abd al-Malik said: It is the voluntary night prayer.