Sunan an-Nasa'i 4192
It was narrated that Yahya bin Husain said:
"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."'
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ يَحْيَى بْنِ حُصَيْنٍ، قَالَ سَمِعْتُ جَدَّتِي، تَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ فِي حَجَّةِ الْوَدَاعِ ‏
"‏ وَلَوِ اسْتُعْمِلَ عَلَيْكُمْ عَبْدٌ حَبَشِيٌّ يَقُودُكُمْ بِكِتَابِ اللَّهِ فَاسْمَعُوا لَهُ وَأَطِيعُوا ‏"‏ ‏.‏