Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) used to deliver the Friday sermon standing, then sit, then stand and deliver the sermon, just as they do today.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5708
Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) says that the Prophet (peace and blessings be upon him) used to deliver two Friday sermons, sitting between them, and he used to deliver both sermons standing.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5709