What is the ruling on one who denies the companionship of Mu'awiyah without reviling him?

Denying the companionship of Mu'awiyah ibn Abi Sufyan - may Allah be pleased with them both - to the Prophet, peace and blessings be upon him, is a grave error. This is a matter agreed upon by the scholars of the Ummah. He was one of the scribes of revelation and narrated Hadiths directly from the Prophet. Whoever denies his companionship after the truth has been made clear to him and insists on denial is mistaken. There is fear of apostasy for anyone who fabricates lies about the Hadiths reported concerning him, such as the Hadith, "Go and call Mu'awiyah for me," and "O Allah, make him a guide, rightly guided, and guide others through him."