Musnad Ahmad 27:1729

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Muhammad ibn Ibrahim ibn al-Harith narrated to me, from Abu Salamah ibn 'Abd al-Rahman ibn 'Awf and Sulayman ibn Yasar—both of them narrated to him—from 'Abdullah ibn 'Umar, who said: "I was with them in the gathering, but I was young, so I did not memorize the hadith." They both said: A man asked him about the Witr, so he mentioned the hadith and said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded that the last of the night prayer be made the Witr."