Fasting: Invalidating a Fast with a Lawful Act Followed by an Unlawful Act
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1639
If a fasting person does something that is lawful but invalidates a fast – for example, he drinks water – and afterwards he does something else that is unlawful and invalidates a fast – for example, he eats unlawful food – then giving one kaffārah is sufficient.