He said to his wife: If you committed zina, you are divorced

Firstly: 

Making divorce conditional upon some unknown matter which cannot be known whether it is true or false is a serious mistake, because it means that the husband is staying with his wife without knowing whether she is permissible for him or not. Hence some of the fuqaha’ said that he should be told to divorce her properly.  

But as far as the ruling is concerned, that may be based on what appears to be the case: so long as she denies that she fell into zina, then divorce does not take place. 

It says in Sharh al-Kharshi ‘ala Khaleel (4/64): If he makes divorce conditional upon some matter that is unknown, then he should be told to separate from her. It was said that it is mustahabb and it was said that it is obligatory but not to be forced by law, such as if he says: You are divorced if you hate me, or if you enter the house, and she says, I will not hate you and I will not enter it, and he does not know if she is telling the truth or lying. End quote. 

This issue on which divorce has been made dependent – if there are no witnesses – cannot be known except from her, so she is responsible for that that, and if she is as her husband says she is, then the divorce takes place, and it is not permissible for her to stay with her husband. 

Secondly: 

What the husband did by making divorce conditional upon that is an obvious mistake. He should have made sure about the matter in some other way. Allaah has prescribed in such cases the procedure of li’aan, if he has strong proof for what he is saying. If he does not want to engage in li’aan there is nothing wrong with him divorcing her (talaaq) instead of living with her when he is in doubt about her chastity. 

But making divorce conditional upon this as he has done will cause her great embarrassment. If she has done what he says she had and admits it, she will be exposing herself, and if she keep quiet, she will be keeping quiet about a serious matter and her living with him will be haraam. 

And Allaah knows best. 

We ask Allaah to set the affairs of the Muslims straight.