What is the expiation for thinking ill of someone who is later proven innocent?

Allah Almighty has forbidden ill thoughts about a believer and prohibited it, as He says: (O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.) [Quran 49:12]. This assumption refers to one whose outward appearance is good and righteous. As for the wicked person who openly commits immorality, it is permissible to think ill of him. Many hadiths have been narrated concerning the prohibition of ill thoughts about a believer, among them the saying of the Messenger of Allah, peace and blessings be upon him: "Beware of suspicion, for suspicion is the falsest of speech." And his saying: "The sanctity of a believer is greater to Allah Almighty than your own sanctity, his wealth and his blood, and that only good be thought of him." As for speaking in accordance with the prohibited assumption, it is a greater sin. It may reach the level of a major sin like slander, or it may be less than a major sin but still forbidden. Repentance is obligatory for it, and seeking forgiveness from the person whose right has been infringed upon.