Musnad Ahmad 16:12

'Abd al-Wahhab al-Thaqafi narrated to us, from Ayyub, from Muhammad, that al-Hasan ibn 'Ali and Ibn 'Abbas saw a funeral procession. One of them stood up, and the other sat down. The one who stood up said, "Did the Messenger of Allah, peace and blessings of Allah be upon him, not stand up?" The one who sat down said, "Yes, but he also sat down."