Muhammad bin Sirin said: Sayyiduna Ibn Abbas and Sayyiduna al-Hasan bin Ali (may Allah be pleased with them both) were sitting when a funeral passed by them. Sayyiduna al-Hasan stood up and Sayyiduna Ibn Abbas remained seated, and he said: "Do you not know that the Messenger of Allah (peace and blessings be upon him) would sometimes stand and sometimes remain seated?"
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2734