What is the ruling for one who breaks their fast believing it is Sha'ban, but later evidence confirms it is Ramadan?
General Chapter
Al-Mughni
Book of Fasting
Primary text
The general consensus of the jurists mandates that such a person must abstain from eating for the remainder of the day and make up the fast (Qada). This ruling is based on the principle that the sanctity of the day remains, as exemplified by Imam Ahmad's ruling that one who engages in sexual intercourse, seeks expiation (Kaffarah), and then repeats the act on the same day remains liable for further expiation because the prohibition of the day has not ended. This is analogous to one who eats believing dawn has not broken when it has actually risen.
Supporting text
A dissenting opinion, attributed to 'Ata and reportedly narrated from Ahmad by Abu al-Khattab (though this narration is doubted), states that the person should eat for the remainder of the day. However, this view is considered weak because the traveler who breaks the fast upon entering a region where it is permissible has a different standing, as their abstention was not required internally, unlike the mistaken faster.