What is the limit for the preferred time of the Fajr prayer?
Chapter on Prayer Times
Al-Mughni
Book of Prayer
Primary text
The preferred time for Fajr lasts until the morning light spreads extensively (Yasfar Al-Nahar), as evidenced by the narration of Jibreel. Any time after this until sunrise is the time of valid excuse and necessity. A person who completes one rak'ah before sunrise is considered to have performed the Fajr prayer in time, based on the consensus derived from the saying of the Prophet (peace be upon him): 'Whoever catches one rak'ah of the morning prayer before the sun rises has caught the morning prayer.'
Supporting text
The companions of the opinion-holders (Ashab Al-Ra'y) argue that if the sun rises while one has only prayed one rak'ah, the prayer is invalidated because it falls into a time prohibited for prayer. This is refuted by the authentic Hadith stating that completing one rak'ah before sunrise counts as having caught the prayer, as obligatory prayers are permitted even in times otherwise prohibited for voluntary prayers.