Fasting Begins Once Fajr/Dawn is Visible 

Shaykh Ibn al-‘Uthaymeen (may Allaah have Mercy on him) explains Allaah’s Order to eat and drink until the light of dawn is distinct from the darkness of night (2:187). The shaykh explains that when a person is unsure if Fajr (dawn) has entered, he may continue eating and drinking, based on the clear instructions in the Verse.
Translated and taught by Moosaa Richardson at the Jaami’ as-Saalimiyyah Mosque in Jeddah, Saudi Arabia (1435/09/20). This clip has been extracted from class #9 of the series, “Quranic Verses About Fasting,” available here on 1MM’s Spreaker channel.
Guidelines regarding ‘prayer timetables’ –