Musnad Ahmad 16:9

'Affan narrated to us, Yazid—meaning Ibn Ibrahim, who is al-Tustari—narrated to us, Muhammad narrated to us, he said: I was informed that a funeral procession passed by al-Hasan ibn 'Ali and Ibn 'Abbas, may Allah be pleased with them both. Al-Hasan stood up, and Ibn 'Abbas remained seated. Al-Hasan said to Ibn 'Abbas: "Did you not see that the Prophet, peace and blessings of Allah be upon him, had a funeral pass by him and he stood up?" Ibn 'Abbas said: "Yes, and he also sat down." Al-Hasan did not deny what Ibn 'Abbas, may Allah be pleased with them both, said.