One of the things that invalidates a fast is lying about Allah, His Messenger, and His... - Jurisprudential Answers - Book of Fasting

Ahmed al-Hasan Response:

Lying is different from denying something or not believing it. What invalidates the fast is lying, not denial, although a person who wrongfully denies may be sinful. Lying concerning a vision or unveiling does not invalidate the fast, although the person is sinful. For someone to be described as lying, the truth must be clear to him and he must deliberately contradict it because of his desire, some personal motive, imitation of another, or the like. A person who is confused, forgetful, or inattentive is not described as a liar.