It is narrated from Sayyiduna Jabir bin Samurah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to deliver the sermon standing, then sit, then stand and deliver the sermon standing. Whoever tells you that he (peace and blessings be upon him) delivered the sermon sitting is a liar. By Allah! I have prayed with him (peace and blessings be upon him) more than two thousand times.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1921