Mukhtasar Bukhari

Narrated from Umm al-Mu'mineen Hafsa (may Allah be pleased with her) that the Messenger of Allah (peace and blessings of Allah be upon him) (had the habit that) when the muezzin would stand to give the call to prayer for Fajr and the Fajr adhan would be given, he would offer two light rak'ahs before the iqamah (takbeer) for the obligatory prayer.