Abu Dawud Book 16, Hadith Number 3322.
Chapter : Not known.
Narated By Abdullah ibn Abbas : A man seized his debtor who owed ten dinars to him. He said to him: I swear by Allah, I shall not leave you until you pay off (my debt) to me or bring a surety. The Prophet (PBUH) stood as a surety for him.
He then brought as much (money) as he promised. The Prophet (PBUH) asked: From where did you acquire this gold? He replied: From a mine. He said: We have no need of it; there is no good in it. Then the Apostle of Allah (PBUH) paid (the debt) on his behalf.