Expiation is binding upon him, even though he did not violate the fast.

Section: If one wakes up having broken his fast believing it to be the month of Sha'ban, then clear evidence (bayyina) of the moon-sighting is established, he is required to abstain [from food and drink for the rest of the day] and make up the fast according to the general consensus of the jurists, except for what was narrated from 'Ata' that he said: "He may eat for the remainder of his day." Ibn 'Abd al-Barr said: "We do not know of anyone who said this other than 'Ata'." Abu al-Khattab mentioned this as a narration from Ahmad, but I do not know of anyone who mentioned it besides him, and I believe this to be incorrect; for Ahmad has explicitly stated the obligation of expiation upon one who has intercourse, then performs expiation, then returns and has intercourse again on the same day, because the sanctity of the day has not departed. If he makes expiation obligatory upon one who is not fasting due to the sanctity of the day, then how could he permit eating? It is not valid to equate this with a traveler who arrives while having broken his fast, and similar cases, because the traveler was permitted to break the fast both outwardly and inwardly, whereas this person was not permitted to break it inwardly; thus, he resembles one who ate while thinking that dawn had not yet broken when it actually had. Once this is established, if he has intercourse during it, he is liable for making up the fast and for expiation, like one who woke up without intending to fast, or one who ate and then had intercourse. If his intercourse occurred before the establishment of the evidence, his ruling is the same as that of one who has intercourse thinking dawn has not broken when it actually had, according to what has already been discussed.

Section: Everyone who breaks their fast while the fast is binding upon them—such as one who breaks it without an excuse, one who breaks it thinking dawn has not broken when it had, one who thinks the sun has set when it has not, one who forgets the intention of the fast, and those similar to them—is required to abstain. We know of no disagreement among them regarding this. However, it is inferred from the opinion of 'Ata' regarding one who is excused in breaking the fast that it is permissible to continue breaking the fast for the rest of the day, by way of analogy to his opinion concerning the situation where evidence of the sighting is established. This is an anomalous (shadh) opinion that the scholars have not paid any attention to.

Section: As for those for whom breaking the fast is permissible at the beginning of the day, both outwardly and inwardly—such as a menstruating woman, a woman in her post-partum period, a traveler, a child, an insane person, a disbeliever, and a sick person—if their excuses cease during the day, such as the menstruating woman or the one in post-partum becoming clean, the traveler arriving, the child reaching puberty, the insane person regaining sanity, the disbeliever accepting Islam, and the sick person who had broken his fast becoming healthy, there are two narrations regarding them. One of them is that they are required to abstain for the remainder of the day. This is the opinion of Abu Hanifah, Al-Thawri, Al-Awza'i, Al-Hasan ibn Salih, and Al-'Anbari, because it is a factor that, if found before dawn, would have necessitated fasting, so if it occurs after dawn, it necessitates abstention, like the establishment of evidence of the sighting.