What is the prescribed time for Fajr prayer?
Chapter on Prayer Times
Al-Mughni
Book of Prayer
Primary text
The time for Fajr prayer begins when dawn truly breaks (Yuzayil al-Rajul Ahla), which is when it becomes forbidden for the fasting person to eat or drink. Mu'adh was instructed to begin Fajr at the beginning of the true dawn during winter, allowing for an extended recitation commensurate with the ability of the congregation without causing fatigue. In summer, it should be delayed until the sky brightens somewhat (Asfar), because the night is short and people are asleep, allowing them time to wake up for it.
Supporting text
Umar stated the time for Fajr is when a man separates from his household (implying the start of the lawful time for prayer/fasting prohibition) and that recitation should be given its due portion.