From Sahih Bukhari (Hadith 5269):
Narrated Abu Huraira:The Prophet (ﷺ) 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.:
Reference: Bukhari 5269