He bought shares in a company then he found out that it deals with riba
Question

                I bought shares thinking that the company does not deal with interest. Then I found out that it has money in banks that deal with interest. What shall I do with these shares?.
Praise be to Allah.
    You have to sell them and get rid of them, and there is no 
    sin on you in that, because you did not know that they are haraam. 

    Shaykh Dr. Muhammad ibn Sa’ood al-‘Usaymi was asked about 
    that and he replied: 
and blessings and peace be upon the 
    Messenger of Allaah and his family and companions. 
    
    If a person deals with a permissible company which then 
    changes into an impermissible company, or he buys it and did not realize 
    that it was haraam, then he comes to know that it is haraam, or he buys it 
    on the basis of a fatwa from scholars who say it is permissible to deal with 
    mixed companies (i.e., those that deal with both halaal and haraam things), 
    then he finds out that this is not permissible, then the ruling is as 
    follows: He has to sell whatever he owns of shares in that company, but he 
    does not have to purify anything; if selling it will cause a loss, then he 
    may wait until he can regain his capital. End quote.