Is expiation (kaffarah) obligatory for someone who repented sincerely and married a religious woman, then was assailed by satanic whispers concerning her, so he went to check on her and found her in the best of conditions, and regretted these whispers and sought Allah's forgiveness?

Among the wife's rights upon her husband is for him to protect her and safeguard her from anything that might tarnish her honor. However, there should be no excessive monitoring of her movements and stillness, nor harboring ill suspicions about her. For the Messenger of Allah, peace and blessings be upon him, said: "Indeed, there is jealousy that Allah loves, and there is jealousy that Allah detests. As for the jealousy that Allah loves, it is jealousy regarding suspicion. And as for the jealousy that Allah detests, it is jealousy without suspicion..." So, if you are certain of her purity and integrity, then be at ease and do not leave room for satanic whispers. And your remorse and seeking forgiveness will lift the blame from you.