Musnad Ahmad 1018:132

Abu al-Nadr narrated to us, Shayban narrated to us, from Ash'ath, from al-Aswad ibn Hilal, from a man of his people, that he used to say during the caliphate of Umar ibn al-Khattab: Uthman ibn Affan will not die until he is made caliph. We said: From where do you know that? He said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "I saw tonight in a dream as if three of my companions were weighed. Abu Bakr was weighed and he balanced, then Umar was weighed and he balanced, then Uthman was weighed and our companion fell short, and he is righteous."