Eating/Drinking When Fajr Time Enters
“This, if it is authentic, is interpreted by the peope of knowledge based on the fact that the Prophet knew that the caller to prayer would also make a call before the actual time for Fajr (translators note: also known as ‘al-Fajr al-Kadhib’ – the False Fajr)…So, this is in reference to the first adhan.”
In ‘al-Umm’ (2/96), ash-Shafi’i said:
“I prefer that the suhur is made late, provided it is not too close to a time in which it is feared that the Fajr has arrived. I like to stop eating at such a time. If the Fajr arrives, and one has something that he has put into his mouth and chewed, then he should spit it out, as entering it into his mouth does nothing to break his fast. Rather, what breaks his fast is his swallowing it.”
So, if one knows that the actual Fajr time has arrived while he is still eating or drinking, he should spit it out. If he cannot prevent himself from swallowing the food or drink, then he is excused due to his inability to do so, and he should not induce vomit, as this itself breaks the fast, as the Prophet said: “Whoever intentionally vomits, then he must make up his fast.”
And Allah Knows best.