Ibn Hibban

Sayyiduna Rafi' bin Khadij narrates this statement of the Prophet (peace and blessings of Allah be upon him): "You people should perform the morning prayer with the morning (light), because whenever you perform the morning prayer with the morning (light), it will be more virtuous for your reward. (The narrator is in doubt, perhaps these are the words) for its reward." (Imam Ibn Hibban, may Allah have mercy on him, says:) Here the Prophet (peace and blessings of Allah be upon him) has commanded to perform the Fajr prayer with brightness. The reason for this is that there is a hidden cause in this command, and that is that the companions of the Prophet (peace and blessings of Allah be upon him) used to start performing the Fajr prayer in darkness. So, on moonlit nights, when a person intends to perform the Fajr prayer at the time of true dawn in darkness, it is possible that his prayer may begin at night (i.e., before the true dawn has appeared). Therefore, the Prophet (peace and blessings of Allah be upon him) commanded to perform the prayer with enough brightness so that it is certain that the true dawn has appeared, and he said: "When you perform the morning prayer," it means that when you are certain that the true dawn has appeared, then this will be a greater cause of reward for you compared to performing the prayer with doubt (i.e., when you are still in doubt about whether its time has begun).

Sayyiduna Rafi' bin Khadij (may Allah be pleased with him) narrates: The Prophet Muhammad (peace and blessings be upon him) said: "Offer the Fajr prayer when it becomes bright, for it is more rewarding."