Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.088A.

The time of the morning prayer begins with the true dawn and lasts until sunrise. It is preferred to pray it early in its permissible time. Abu Mas’ud al-Ansari reported that the Messenger of Allah prayed the morning prayer in the darkness (of the dawn). Another time, he prayed it when the dawn was shining (or glowing). Then after that, he always prayed in the darkness (of the dawn) until he died.” (Related by Abu Dawud and al-Baihaqi. Its chain is sahih.) Said ‘Aishah, “Believing women would pray the morning prayer with the Prophet, upon whom be peace, being enveloped in their clothing. They would return to their homes after the prayer and no one could recognize them due to the darkness (of the dawn).” (Related by “the group.”)

Rafa’ ibn Khadeej related a hadith in which the Prophet, upon whom be peace, said, “Make the morning prayer at daybreak, as your reward will be greater.” In another version it states, “Make the morning prayer at the shining (time of the dawn), as your reward will be greater.” (Related by “the five.” At-Tirmizhi and Ibn Hibban grade it as sahih.) It refers to the time that one finishes the prayer, not the time when one begins it. That is, one should make the recital long so that one is in prayer until the dawn becomes “shiny.” This is what the Prophet, upon whom be peace, used to do, for he would recite between 60 and 100 verses. It also means to make sure that the dawn has come.