What is the punishment in islam if wife beats her...

Praise be to God, and prayers and peace be upon the Messenger of Allah, and on his family and companions and those who follow him 
Please note that married life must be based on affection and mercy, and on cohabitation with virtue, and that the guardianship for men, and it is not permissible for a man to beat his wife, beating only within the limits that’s allowed by Islam.
It is not permissible for a woman to infringe upon her husband by beating, at all. This is a great evil, and it is a violation of the man's authority which is a great sin.

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