Sahih Bukhari Volume 003, Book 048, Hadith Number 852.
Narated By Um Al-Ala : That when the Ansar drew lots as to which of the emigrants should dwell with which of the Ansar, the name of Uthman bin Mazun came out (to be in their lot). Um Al-Ala further said, "Uthman stayed with us, and we nursed him when he got sick, but he died. We shrouded him in his clothes, and Allah's Apostle came to our house and I said, (addressing the dead 'Uthman), 'O Abu As-Sa'ib! May Allah be merciful to you. I testify that Allah has blessed you.' The Prophet said to me, "How do you know that Allah has blessed him?" I replied, 'I do not know O Allah's Apostle! May my parents be sacrificed for you.' Allah's Apostle said, 'As regards Uthman, by Allah he has died and I really wish him every good, yet, by Allah, although I am Allah's Apostle, I do not know what will be done to him.' Um Al-Ala added, 'By Allah I shall never attest the piety of anybody after him. And what Allah's Apostles said made me sad." Um Al-Ala further said, "Once I slept and saw in a dream, a flowing stream for Uthman. So I went to Allah's Apostle and told him about it, he said, 'That is (the symbol of) his deeds."