Musnad Abu Ya'la al-Mawsili 3:6

Abu Khaythama narrated to us, Abu Mu'awiyah narrated to us, Layth narrated to us, from Mujahid, from 'Abdullah ibn Sakhbara, who said: A funeral procession passed by 'Ali, and his companions went to stand up. 'Ali said to them, "What compels you to do this?" They said, "Abu Musa informed us that the Messenger of Allah (peace be upon him) used to stand up when a funeral procession passed by him until it passed him." He said, "Abu Musa does not say anything [wrong], but perhaps the Messenger of Allah (peace be upon him) did that once. The Messenger of Allah (peace be upon him) used to like to resemble the People of the Book in matters where nothing had been revealed to him. But when it was revealed to him, he abandoned it."