Musannaf Ibn Abi Shayba 3:4373

Abu Bakr narrated to us, he said: Salam bin Sulaym narrated to us, from Abu Ishaq, from al-Harith, from Ali. And Sharik narrated to us, from Abu Ishaq, from al-Harith, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to perform the Witr prayer at the time of the Adhan, and he would pray the two rak'ahs with the Iqamah. Salam added: "The first Adhan." Salam said: And I heard Abu Ishaq say one time: "He would perform the Witr prayer at the break of dawn."