Sahih Al Bukhari – Book of Divorce
Volumn 007, Book 063, Hadith Number 194.
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Narated By Abu Huraira : The Prophet said, "Allah has forgiven my followers the evil thoughts that occur to their minds, as long as such thoughts are not put into action or uttered." And Qatada said, "If someone divorces his wife just in his mind, such an unuttered divorce has no effect.