Shaykh Muhammad ibn Saaleh Al Uthaymeen may Allah have mercy upon him was asked:
“what is the ruling on the one who eats while in doubt if the sun has risen for fajr or not then after that it becomes clear to him that the sun did actually rise, and and also the one who eats thinking the sun has set then after that it becomes clear to him that the sun did not actually set?”
The noble Shaykh replied by saying:
“if one is in doubt and thinks – has the sun risen or not – then he eats and then after that it becomes clear to him that the sun already rose then it is not upon him to make up his fast; even if he is completly unsure whether it has risen or not. This is because Allah the most high says “ and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of the night)”…………
And eating in this time is permitted and there is not no sin on the one who eats in this time nor does he have to make up his fast.As for if one eats thinking the sun has set and then it becomes clear to him after that the the sun did not actually set then it is not upon him to make up his fast upon the most correct opinion, because of the hadeeth of Asmaa bint abu Bakr: the companions opened their fast in the time of the prophet on a cloudy day and they were not orderd to make up their fast.
As for the one who eats in doubt whether the sun has set and then after that it becomes clear to him that the sun did not set then it is obligatory that he makes up his fast, beause he eate in this state, meaning in a state of doubt and it is haraam for one to eat whilst being in this state of doubt. Rather it is not permissable for him to open his fast except if he is certain that the sun has set or at least he is leaning more towards that. In this particular state, meaning if he opens his fast and he is in doubt then it becomes clear to him that the sun did not set it is waajib for him to repeat his fast because his was not permitted to open his fast in that state.
taken from the majoo alfatawa wa rasaa’il: fatwa number 263
As recieved from west_london_dawah yahoo groups