Zhihar (a form of separation between a husband and wife until expiation is fulfilled. It signifies the man's likening of his wife to a kinswoman unmarriageable to him. The usual form is: You are to me as my mother's back) was a type of divorce in the Pre-Islamic Era, and it was said that it existed in the early beginning of Islam. It was also said: In the Pre-Islamic Era, when someone hated his wife and did not want someone to marry her, he would practice Ila' or Zhihar to keep her without a husband and without being free to marry another. So, the Lawgiver changed the ruling to the prohibition of Zhihar and paying the penance.
Its form is: Likening the wife who is not irrevocably divorced with a female who is not permissible for a person to marry. This means likening the lawful female [i.e., the wife] with unlawful female, therefore Allah (Glory be to Him) described it as a deniable saying and a kind of falsehood. Allah (Glorified and Exalted be He) said in the Ayah of Zhihar: "Those among you who make their wives unlawful to them by Zihâr (i.e. by saying to them "You are like my mother's back,") they cannot be their mothers. None can be their mothers except those who gave them birth. And verily, they utter an ill word and a lie. And verily, Allâh is Oft-Pardoning, Oft-Forgiving. * And those who make unlawful to them their wives by Zihâr and wish to free themselves from what they uttered, then (the penalty in that case is) the freeing of a slave before they touch each other. That is an admonition to you (so that you may not repeat such an ill thing). And Allâh is Well-Acquainted with what you do. * And he who finds not (the money for freeing a slave) must fast two successive months before they both touch each other. And he who is unable to do so, should feed sixty Miskîn (needy persons). That is in order that you may have perfect Faith in Allâh and His Messenger. These are the limits set by Allâh. And for disbelievers, is a painful torment." (2)
(1) See: Bada'i` As-Sana'i` 3/299, Al Bahr Ar-Ra'iq 6/308, Al Mudawanah 2/309, Al Majmu` Sharh Al Muhadhdhab 17/343, Mughny Al Muhtaj 3/352, Al Mughny 8/560, Hashiyat Ar-Rawd Al Murbi` 6/628, and Al Insaf9/140.
(2) Surat Al Mujadalah: 2-4.