Whoever intends without doubt that he will break fast, then his fasting is immediatly broken.
Shaykh Ibn ‘Uthaymeen was asked about a man who was travelling and fasting in Ramadaan, and intended to break his fast but then he did not find anything with which to break his fast, so he changed his mind and completed the fast until Maghrib. Is his fast valid or not?
His fast is not valid and he has to make it up, because when he intended to break the fast, he broke his fast. But if he had said ‘If I find water I will drink, otherwise I will carry on fasting,’ and he did not find any water, then his fast is valid, because his intention was not definite, rather breaking the fast was conditional upon finding something, but he did not find that thing, so his initial intention remained in effect.
And explained in Zaad al-Mustaqni’:
The one who intends to break the fast has broken the fast. And I warn you, my brothers, against those who are not well versed in knowledge; I warn you against them if they say that they do not know of anyone who said that, or that no one said that. They may be telling the truth, because they do not know the books of the scholars and they have not studied them, and they do not know anything about them. Moreover, even if we assume that this is not to be found in the books of the scholars, didn’t the Prophet (peace be upon him) say: “Actions are but by intentions”? So if he said “Actions are but by intentions” and this man intended to break his fast, he has broken his fast.
Liqaa’aat al-Baab il-Maftooh (29/20).