Mukhtasar Muslim

Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings of Allah be upon him) would deliver the sermon standing. Then he would sit down, then stand up and deliver it standing. And whoever among you says that he used to deliver it sitting, by Allah, he has lied. And by Allah, I have offered more than two thousand prayers with the Prophet (peace and blessings of Allah be upon him).