Shaykh Ibn Baaz (may Allaah have Mercy on him) said:

"It is well known that people who live in well-lit (large) cities can not determine the time of the entrance of Fajr by their own sighting (of the horizon).  Thus, it is upon them to be cautious and go by the athaan and by prayer schedules that determine the times of Fajr's entrance according to the hour and minute, acting by the statement of the Messenger of Allaah (sallallaahu 'alayhe wa sallam): "Leave what makes you doubt for that which does not make you doubt."  And his statement (sallallaahu 'alayhe wa sallam): "Whoever stays away from doubtful matters will keep his religion and his honor protected."  And Allaah is the Granter of success. 
 

Source: Majmoo' al-Fataawee of Shaykh Ibn Baaz (15/286), may Allaah have Mercy on him

Courtesy: http://www.salafitalk.net/st/viewmessages.cfm?Forum=16&Topic=7337