Musnad Ahmad 3:55

Hassan ibn Musa narrated to us, Abdullah ibn Lahi'ah narrated to us, Abu Qubayl narrated to us, he said: I heard Malik ibn Abdullah al-Zabadi narrating from Abu Dharr: That he came seeking permission to enter upon Uthman ibn Affan, and he was granted permission, and in his hand was his staff. Uthman said: "O Ka'b, Abd al-Rahman has passed away and left behind wealth, so what is your opinion regarding it?" He said: "If he was fulfilling the right of Allah regarding it, then there is no harm upon him." Abu Dharr raised his staff and struck Ka'b, and said: "I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: 'I would not love for this mountain to be gold for me, that I spend it and it be accepted from me, while I leave behind me six uqiyahs.' I adjure you by Allah, O Uthman, did you hear it?" — three times. He said: "Yes."