Zihar

Zihar
If any men among you divorce their wives by Zihar (calling them mothers), they cannot be their mothers: None can be their mothers except those who gave them birth. And in fact they use words (both) iniquitous and false: but truly Allah is one that blots out (sins), and forgives (again and again).
( سورة المجادلة , Al-Mujadila, Chapter #58, Verse #2)

But those who divorce their wives by Zihar, then wish to go back on the words they uttered,- (It is ordained that such a one) should free a slave before they touch each other: Thus are ye admonished to perform: and Allah is well-acquainted with (all) that ye do.
( سورة المجادلة , Al-Mujadila, Chapter #58, Verse #3)