Purity: Divorce and Intercourse During Nifas
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 503:
A divorce given to a woman who is in the state of nifās is invalid, and having sexual intercourse with a woman who is in the state of nifās is unlawful but it does not require one to give kaffārah.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)