Musnad Ahmad 5:62

Isma'il ibn Ibrahim narrated to us, from Muhammad ibn 'Amr, who said: Waqid ibn 'Amr ibn Sa'd ibn Mu'adh narrated to me, saying: I attended a funeral among the Banu Salama, so I stood up. Nafi' ibn Jubayr said to me: "Sit down, for I will inform you of something established regarding this: Mas'ud ibn al-Hakam al-Zuraqi narrated to me that he heard 'Ali ibn Abi Talib at the Rahba of Kufa, and he was saying: "The Messenger of Allah, may Allah bless him and grant him peace, commanded us to stand for the funeral, then he sat after that and commanded us to sit."