Prayer: Establishing the Onset of Prayer Time
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 729
One can start performing prayers when he attains certainty (yaqīn) that the time has set in or when two dutiful men inform him that the time has set in. In fact, one can conclude that the time for the morning prayer has set in if he hears the adhān said by someone whom he knows is extremely careful in observing the time of prayers, or if he is informed by such a person, provided that he derives confidence (iṭmiʾnān) from it.