Musnad Ahmad 16:11

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ayyub, from Ibn Sirin, that a funeral procession passed by Ibn Abbas and al-Hasan ibn 'Ali. One of them stood up, and the other remained seated. The one who stood up said, "Do you not know that the Messenger of Allah, peace and blessings of Allah be upon him, stood up?" He replied, "Yes, and he also sat down."