Is it permissible to say that Mu'awiya cursed the Messenger of Allah, Allah bless him and grant him peace, because he and his commanders cursed Ali, may Allah honor his face?

Mu'awiyah did not revile Ali, nor did he order it, as there is no authentic chain of narration to prove that. What was narrated from him was merely a question about the reason for Sa'd ibn Abi Waqqas's refusal to revile Abu Turab, and this is not an explicit statement of revilement. Moreover, historians transmit both good and bad, and they do not distinguish between what is authentic and what is weak. What is known about Mu'awiyah is his preservation of Ali's virtue, and this has been proven in the story of Abu Muslim al-Khawlani, Abu al-Darda', and Abu Umamah, may Allah be pleased with them. Mu'awiyah was also famous for his forbearance and forgiveness; he used to pardon those who publicly reviled him. Those who disparage Mu'awiyah are harming the Prophet, may Allah's peace and blessings be upon him, for appointing him as a scribe of revelation, and they are harming the Companions and Al-Hasan ibn Ali, may Allah be pleased with them, for his concession of the caliphate to Mu'awiyah.