I believe he traced it back to the Prophet ﷺ, who said, "Before the Hour there will be days of harj, days in which knowledge will vanish and ignorance will spread."Abu Musa said, "Harj, in the language of the Abyssinians, means killing."
حدثنا محمد بن جعفر حدثنا شعبة عن واصل عن أبي وائل
وأحسبه رفعه إلى النبي ﷺ أنه قال بين يدي الساعة أيام الهرج أيام يزول فيها العلم ويظهر فيها الجهل فقال أبو موسى الهرج بلسان الحبش القتل
Its isnad is sahih, and it is a repetition of 3695, 3817, and 3841, but in all of those it is narrated from Ibn Masud and Abu Musa Ashari, and the narrators there stated categorically that it goes back to the Prophet, without any doubt as Shubah doubted here. The apparent sense of those narrations is that the tafsir of "harj" is also attributed to the Prophet, but this narration indicates that it is part of Abu Musa's own words, and perhaps it is the part that Shubah doubted in attributing to the Prophet.