It is narrated from Hazrat Jabir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to deliver the sermon while standing.
Hadith Referenceمسند احمد / أول مسند البصريين / 20868
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن