Mu'adh bin Mu'adh narrated it from Shu'bah, and in this hadith he said: A slave was leading them in prayer. They said to Abu Dharr (may Allah be pleased with him): "Go forward." He refused, so the slave went forward and led them in prayer. Then he mentioned the hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5120