Musnad Ahmad 5:98

Ibrahim ibn Abi al-Abbas narrated to us, Sharik narrated to us, from Abu Ishaq, from al-Harith, from Ali, from the Prophet, peace and blessings of Allah be upon him, who said: "He used to perform the Witr prayer at the time of the Adhan, and he would pray the two rak'ahs at the time of the Iqamah."