All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
A wife saying to her husband "You are forbidden for me" is idle talk since the lawful is what Allah Has Made lawful and the unlawful is what Allah Has Made unlawful and no human being has the right to change the Judgments of Allah, and the above wife doesn`t have to pay Kaffarah (Expiation) although she shouldn`t have uttered those words since, in principle, a Muslim should avoid idle talk. The evidence on this is that Almighty Allah Says {What means}: " Who avoid vain talk;" {Al-Mu`minun, 3}.
However, if the husband said that statement to his wife without intending talaq (Divorce pronounced by a husband) nor Zihar (a man likening his wife to an unmarriageable relative), but to forbid for himself having intercourse with her, then he must pay Kaffarah like that of an oath since Almighty Allah Says: "O Prophet! Why holdest thou to be forbidden that which God has made lawful to thee? Thou seekest to please thy consorts. But God is Oft-Forgiving, Most Merciful. God has already ordained for you, (O men), the dissolution of your oaths (in some cases): and God is your Protector, and He is Full of Knowledge and Wisdom." {At-Tahrim, 1-2}. This verse was revealed when the Prophet (PBUH) forbade for himself having intercourse with Maria al-Qibtiyya. It was narrated from Anas: "that the Messenger of Allah had a female slave with whom he had intercourse, but 'Aishah and Hafsah would not leave him alone until he said that she was forbidden for him. Then Allah, The Mighty and Sublime, Revealed: "O Prophet! Why do you forbid (for yourself) that which Allah has allowed to you.' until the end of the Verse." {Related by An-Nasa`i}.
The difference between the husband saying the above words to his wife and the wife saying them to him is that the husband has the right to forbid his wife for himself by divorcing her. Thus, his words are like an oath with which he threatens his wife, so Sharia, in this case, commands him to pay an expiation. As for the wife, she doesn`t have the right to divorce her husband, so she doesn`t have to pay Kaffarah. And Allah The Almighty Knows Best.