Musannaf Ibn Abi Shayba 3:3957

Abu al-Ahwas and Sharik narrated to us, from Abu Ishaq, from al-Harith, from Ali, that the Prophet (peace and blessings of Allah be upon him) used to pray two rak'ahs at the [time of the] adhan. One of them said: And he would perform witr at the [time of the] iqamah.