What is the exact time one must stop eating and drinking for fasting, and what should someone do if they were drinking at the time of the Adhan?

The obligation in fasting is to abstain from all things that break the fast from the rise of true dawn until sunset. If a Muslim knows of the rise of true dawn by observation, or by the information of another, or hears an adhan (call to prayer) that is made on time, he must immediately abstain. Some scholars have made an exception for the case where a person has a vessel in their hand when hearing the adhan, in which case they are permitted to finish their need from it. However, it is more proper and safer to abstain upon hearing the adhan, especially given the difficulty of ascertaining the true dawn in modern cities.