Musnad Abu Ya'la al-Mawsili 160:60

Yahya ibn Ayyub narrated to us, Ismail ibn Ja'far narrated to us, from al-Ala', from his father, that he witnessed a funeral, and Marwan ibn al-Hakam prayed over it. Abu Hurayrah went [away], and we sat in the graveyard. Then Abu Sa'id came and said to Marwan, "Show me your hand." So he gave him his hand, and he said, "Stand up." He said, "So he stood up." Then Marwan said, "Why did you make me stand up?" He said, "The Messenger of Allah (peace be upon him) used to stand when he saw a funeral until it passed by him, and he said, 'Indeed, death is a terror.'" Marwan said, "Do you confirm this, Abu Hurayrah?" He said, "Yes." He said, "Then what prevented you from telling me?" He said, "You were the Imam, so you sat, and I sat."