Musnad Ahmad 27:1041

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Anas ibn Sirin, who said: I asked Ibn ‘Umar, "What should I recite in the two rak‘ahs before the morning [prayer]?" Ibn ‘Umar said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray at night two by two, and he would perform witr with one rak‘ah at the end of the night." Anas said: I said, "But I am asking you what I should recite in the two rak‘ahs before the morning [prayer]?" He said: "Bah, bah! You are indeed persistent. I am only narrating to you," or he said, "I am only relating to you the hadith: 'The Messenger of Allah (peace and blessings of Allah be upon him) used to pray at night two rak‘ahs by two rak‘ahs, then he would perform witr with one rak‘ah at the end of the night, then he would stand as if the adhan or the iqamah were in his ears.'"