Simak says: I heard Sayyiduna Jabir bin Samurah (may Allah be pleased with him) say: "I saw the Prophet (peace and blessings be upon him) deliver the sermon standing on Friday, then he (peace and blessings be upon him) would sit and not speak, then stand and deliver another sermon. So whoever tells you that the Messenger of Allah (peace and blessings be upon him) delivered the sermon sitting has lied."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1898