Abu Dawud Book 2, Hadith Number 1252.

Chapter : On observing the sunan prayer at dawn lightly.

Narated By Bilal : Ziyadah al-Kindi reported on the authority of Bilal that he (Bilal) came to the Apostle of Allah (PBUH) to inform him about the dawn prayer. ‘Aisha kept Bilal engaged in a matter which she asked him till the day was bright and it became fairly light. Bilal then stood up and called him to prayer and called him repeatedly. The Apostle of Allah (PBUH) did not yet come out. When he came out, he led the people in prayer and he (Bilal) informed him that ‘Aisha had kept him engaged in a matter which she asked him till it became fairly light; hence he became late in reaching him (in time). He (Bilal) said: Apostle of Allah, the dawn became fairly bright. He said: If the dawn became brighter than it is now, I would pray them (the two rak’ahs of the sunnah prayer), offer them well and in a more beautiful manner.