The Prophet’s Funeral Prayer for the Negus

📖 Sahih Muslim 951 a
The Messenger of Allah ﷺ gave the people news of the death of Negus on the day he died, and he took them out to the prayer place and observed four takbirs.”

In another narration:

📖Sunan an-Nasai 1970
It was narrated from Jair that: the Messenger of Allah said: “Your brother An-Najashi has died, so get up and offer the funeral prayer for him”. He stood up and put us in rows as is done for the funeral prayer, and we prayed for him.

👉 He didn’t say:
“Let’s respect the king.”
He said: “Your brother” and did Salat al-Janazah, which always includes asking Allah to forgive the deceased.

That means it was a prayer only suitable for a believer.

📖Quran 9:113
It is not for the Prophet or the believers to ask forgiveness for the polytheists, even if they are close relatives…”

Summary:

Najashi was a Muslim, though his people didn’t know it, The Prophet ﷺ offered Salat al-Janazah for him, not out of political respect, but because he died as a believer.