The ruling on the Fast of whoever ate while being uncertain as to whether Fajr time had begun or whether the Sun had set
Question :
What is the ruling on the fast of someone who ate or drank while being uncertain about whether Fajr time had began or whether the sun had set? Please benefit us with an answer, and may you be rewarded (by Allâh).
Answer:
Whoever ate or drank while being uncertain about the entry of Fajr time, there is no sin on him and his fasting is correct. This is because the basic principle is that it still remains night (until proven otherwise). What is legislated for the believer is that he eats Sahur (predawn breakfast) before the time of doubt to protect his religion (from doubt) and to strive to perfect his fasting.
In reference to whoever ate or drank while being uncertain about the setting of the sun, then he has erred and he must make up for that day of fasting. This is because the basic principle is that it still remains daytime (until proven otherwise). Therefore, it is not permissible for the Muslim to break his fast unless he is certain that the sun has set or he feels certain that it probably has set. And Allâh is the Giver of success.
Source:
Ash-Shaykh Ibn Baz
Fatawa Islamiyah, Vol. 3 Pages 256-257