Musnad Abu Ya'la al-Mawsili 154:139

Abu al-Rabi' narrated to us, Hammad narrated to us, Anas ibn Sirin narrated to us, he said: I asked Ibn Umar, I said: "The two rak'ahs before the dawn prayer, should I lengthen the recitation in them?" He said: "The Messenger of Allah (ﷺ) used to pray at night two by two, and he would perform witr with one rak'ah." I said: "It is not about this that I am asking you." He said: "You are indeed persistent! Will you not let me recite the hadith to you? The Messenger of Allah (ﷺ) used to pray at night two by two, and he would perform witr with one rak'ah, then he would pray two rak'ahs before the dawn, while the adhan was in his ears."