Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that when the Prophet (peace and blessings be upon him) came out on Friday and sat on the minbar, Bilal (may Allah be pleased with him) would then give the adhan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5746