"I heard my grandmother say: 'I heard the Messenger of Allah say, during the Farewell Pilgrimage: If an Ethiopian slave is appointed over you who rules according to the Book of Allah, then listen to him and obey."' (Using translation from Nasāʾī 4192)  

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ فِي حَجَّةِ الْوَدَاعِ «وَلَوِ اسْتُعْمِلَ عَلَيْكُمْ عَبْدٌ حَبَشِيٌّ يَقُودُكُمْ بِكِتَابِ اللهِ فَاسْمَعُوا لَهُ وَأَطِيعُوا»