The time when one must stop eating and drinking — the beginning of the fast — is the imsāk (إِمْسَاك) time, which coincides with the start of the Fajr prayer.
If the Fajr adhān (morning call to prayer) is called after the entry of imsāk time, it is not permissible to continue eating or drinking after that point.
If a person mistakenly believes that they may continue eating after imsāk and does so, their fast is invalid, and they must make up (qaḍāʾ) that day’s fast.
To avoid any doubt or risk of invalidating the fast, one should stop eating and drinking before the adhān begins.