Musnad Ahmad 27:412

Yazid narrated to us, Shu'bah informed us, from Anas ibn Sirin, from Abdullah ibn Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray the night prayer two by two, then he would perform witr with one rak'ah at the end of the night. Then he would stand up as if the adhan or the iqamah were in his ears."