Sayyiduna Abdullah bin Umar (may Allah be pleased with them) narrated that the Prophet Muhammad (peace and blessings be upon him) used to deliver the sermon while standing, then he would sit down, and then stand up again, just as you people do nowadays.
Hadith Referenceاللؤلؤ والمرجان / كتاب الجمعة / 499
Hadith Gradingمحدثین:«صحیح»
Hadith Takhrij«صحیح، أخرجه البخاري في: 11 كتاب الجمعة: 27 باب الخطبة قائما»