The Sunnah is that that neither could a kaafir inherit from a Muslim, nor a Muslim inherit from a kaafir. The first person to allow a Muslim to inherit from Kafir, whilst Kafir could not inherit from a Muslim was Muawiya, and Bani Umaya did the same after him till Umar bin Abdulaziz came and revived the Sunnah, but then Hisham returned back to what Mu’awiya and Bani Umatya used to do.
Muhammad bin Ka’ab al-Qurdhi said: ‘Abdulrahman bin Sahl al-Ansari
participated in a war during Uthman’s reign and Mu’awiya was a ruler of Syria, then a barrel of alcohol passed before him (Abdulrahman), so he went there while holding his spear and penetrated
into every barrel, the slaves resisted him, till Mu’awiya was informed about that. (Mu’awiya) said: “Leave him, he is an old man and has lost his mind’. (Abdulrahman) said: ‘By Allah, he has
lied, I didn’t lose my mind, but the messenger of Allah (pbuh) forbade us to drink it, I swear by Allah that if I live till I see what I heard from the Messenger of Allah about Mu’awiya, either I
will split and open Muawiya’s stomach or I will die’.
Kanzul Ummal, Volume 5 page 713 Hadith 13716
Muawiya not only made changes to the Eid prayers but he also had the audacity to make changes in the daily prayers by reducing the Takbeer. Imam of Ahle Sunnah Badruddin al Ayyni records in ‘Umadatul Qari Sharah Sahih Bukhari’ Volume 6 page 58:
Mustadrak al-Hakim, Volume 1 pages 364-365
Sharah Sunan Nasai
“He didn’t name the chapter as ‘virtues’ and ‘merits of Mu’awyia’ as a condemnation”.
“One party of people fabricated traditions about the merits of Muawiya and
they narrated hadith from the prophet (pbuh) in that matter (Muawiya’s merits) all of which are lies.”
Minhaj al Sunnah, Volume 4 page 400