Sayyiduna Jabir bin Samurah (may Allah be pleased with him) said: I saw the Messenger of Allah (peace and blessings be upon him) delivering the sermon standing; then he (peace and blessings be upon him) would sit for a short while in silence, then stand and deliver a second sermon on his pulpit. Whoever tells you that he saw him (peace and blessings be upon him) delivering the sermon sitting, do not believe him.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1960