Musnad Ahmad 5:8

Abu Sa'id and Husayn ibn Muhammad narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from al-Harith, from Ali, who said: "The Messenger of Allah, may Allah bless him and grant him peace, used to pray the two rak'ahs of Fajr at the time of the iqamah."