Fasting: Intentionally Invalidating a Fast Mistakenly Believing It Is Ramadan
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1645
If a person is certain that it is the first day of the month of Ramadan and he intentionally invalidates his fast, and afterwards it becomes known that it was actually the last day of Shaʿbān, kaffārah is not obligatory for him.