When the Negus died, the Prophet ﷺ said, "Seek forgiveness for him."
أخبرنا قتيبة قال حدثنا سفيان عن الزهري عن أبي سلمة
لما مات النجاشي قال النبي ﷺ استغفروا له
No references are available.
[Its chain of transmission is sahih]. Ahmad narrated it, 2/241, and Humaydi, no. 1029, from Sufyan bin Uyayna, from Zuhri, with this wording. Both of them stated explicitly that they had heard it, as found in Humaydi, and it is also in Kubra, no. 2168.