What is the original time to abstain from food during fasting: is it Fajr, or ten minutes before it?

The time for the legal abstention from things that break the fast is the rise of the true dawn, as Allah Almighty says: "And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]." What some people do by abstaining ten minutes or so before dawn, out of precaution, has no basis in Islamic law and no validity.