Musnad Ahmad 1153:1918

Ibn Numayr narrated to us, he said: Hisham narrated to us, from his father, from Aisha, she said: "The prayer of the Messenger of Allah, peace and blessings of Allah be upon him, at night was thirteen rak'ahs. He would perform witr from that with five, not sitting in any of them except in the last one. Then, when the mu'adhdhin gave the adhan, he would stand and pray two light rak'ahs."