Al-Zuhri (may Allah have mercy on him) said: It reached us that the first Friday prayer was held before the Prophet (peace and blessings be upon him) came to Madinah; Mus'ab bin Umayr (may Allah be pleased with him) led the Muslims in the Friday prayer. And it also reached us that there is no Friday prayer except with a sermon; whoever does not deliver the sermon should pray four rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5702