Abu Dawud Book 24, Hadith Number 3955.
Chapter : Which of the slaves is better that should be emancipated.
Narated By Marrah ibn Ka’b : Amr ibn Abasah, said that Marrah ibn Ka’b said: I heard the Apostle of Allah (PBUH) say: If anyone emancipates a Muslim slave, that will be his ransom from Jahannam.