Many a times, Muslims even after the Fajr Adhaan has started yet continue eating and they think that the end of Suhoor time is only at the end of the Fajr Adhaan. In fact, the moment the Fajr Adhaan starts, the moment the beginning of Dawn starts, the Suhoor time ends. So this is a common error which nullifies or invalidates the fast.

Dr Zakir Naik