It is narrated from Hazrat Jabir (may Allah be pleased with him) that I saw the Prophet (peace and blessings be upon him) delivering the sermon while standing. Therefore, if anyone among you says that he saw the Prophet (peace and blessings be upon him) delivering the sermon while sitting, he is not telling the truth. The Prophet (peace and blessings be upon him) used to deliver two sermons: first, he would deliver one sermon and then sit down, then stand up and deliver the second sermon.
Hadith Referenceمسند احمد / أول مسند البصريين / 20865
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن