Musnad Abu Ya'la al-Mawsili 252:9

Ishaq narrated to us, Abd al-Razzaq narrated to us, Ma'mar informed us, from Abu Hazim, from Sahl ibn Sa'd, that Uhud shook while the Messenger of Allah (peace and blessings of Allah be upon him), Abu Bakr, Umar, and Uthman were upon it. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Be firm, O Uhud, for there is upon you nothing but a Prophet, a Siddiq (truthful one), or two martyrs."