It is narrated from Qays bin Abi Hazim that he said: We came to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), and he said: Who are your muezzins? We said: Our slaves and our freed slaves (mawali). He gestured with his hand like this, that is, turning it over: Our slaves and our mawali! This is a great deficiency in you. If I had the strength to give the adhan along with being caliph, I would give the adhan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2002