Musnad Ahmad 1726

Yazeed - i.e., Ibn Ibraheem, who is at-Tustari - narrated to us: Muhammad narrated to us: I was told that a funeral passed by al-Hasan bin "Ali and Ibn "Abbas (رضي الله عنه). Al-Hasan stood up and Ibn "Abbas remained sitting. Al-Hasan said to Ibn "Abbas: Did you not see when a funeral passed by the Prophet (ﷺ) and he stood up? Ibn Abbas said: Yes, and he sat. al-Hasan did not object to what Ibn "Abbas said.