Musnad Ahmad 5:634

'Abd al-Razzaq narrated to us, Sufyan informed us, from Layth, from Mujahid, from Abu Ma'mar, who said: We were with 'Ali, and a funeral passed by him, so some people stood up for it. 'Ali said: "Who gave you a fatwa for this?" They said: "Abu Musa." He said: "The Messenger of Allah (ﷺ) only did that once, and he was imitating the People of the Book. When he was forbidden [from doing so], he stopped."