What is the expiation for someone who, unintentionally and while angry, insulted an entire people or a city and its inhabitants?

Slandering and cursing a Muslim without right is forbidden, and its perpetrator is a transgressor (fasiq). The expiation for this is to seek their forgiveness and pardon. The Prophet (peace be upon him) warned against speaking without verification, as one might fall into Hellfire because of it. Slandering an entire people or city without right is impermissible. The expiation for this is to frequently supplicate and seek forgiveness for those one slandered, and to praise them for the good one knows in them.