Allama Shihabuddin Ahmad bin Mohammad al Khafaaji al Hanafi رحمہ اللہ (d. 1069 A.H) said:
ومن يكون يطعن في معاوية فذلك كلب من كلاب الهاوية
One who abuses Saydna Ameer Muawiya رضیﷲ عنه then he's a dog from the dogs of hell.
[Naseem-Ur-Riyaz Fi Sharah Ash-Shifa Al-Qazi Ayaaz Li Khafaaji]
Via Huzaifa Mohammad
