It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that a man asked the Prophet (peace and blessings be upon him) about the time of the dawn (Fajr) prayer. The Messenger of Allah (peace and blessings be upon him) prayed the dawn prayer in the darkness, then the next day he prayed it when it was well lit, then said: "Where is the one who asked?" He said: Here I am. He (peace and blessings be upon him) said: "The time is between these two."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13721