Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1562
Ibn 'Umar reported that Hafsa, the Mother of the Believers, informed him that when the Mu'adhdhin became silent after calling (people) to the dawn prayer, the Messenger of Allah (may peace be upon him) commenced the dawn (prayer) when it dawned by observing two short rak'ahs before the commencement of the (Fard) prayer.