Sahih Bukhari
Manumission of Slaves
Volume: 003 / Book Number: 046 / Hadith Number: 714
Narrated Anas:
Some men of the Ansar asked for the permission of Allah's Apostle and said, "Allow us to give up the ransom from our nephew Al-'Abbas. The Prophet said (to them), "Do not leave (even) a Dirham (of his ransom)."