Abdullah bin al-Samit says that Abu Dharr (may Allah be pleased with him) went to al-Rabadhah, and at the watering place there was an Abyssinian slave. When the iqamah for the prayer was called, Abu Dharr (may Allah be pleased with him) was asked to lead the prayer, and the slave stepped back. Abu Dharr (may Allah be pleased with him) said to him: Step forward, for my dear friend (peace and blessings be upon him) enjoined me "to listen and obey, even if the leader is an Abyssinian slave with cut ears."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5854