Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "He who sets free a Muslim slave, Allah will deliver from the fire of Hell every limb of his body in return for every limb of the slaves body, even his private parts."[Al-Bukhari and Muslim].
It was the result of such instructions that the Companions of the Prophet (PBUH) did their best for the liberation of slaves. Abu Bakr As-Siddiq (May Allah be pleased with him) bought many slaves and set them free. `Abdur-Rahman bin `Auf released as many as thirty thousand slaves. `Abdullah bin `Umar liberated more than a thousand of them. It is stated that some Companions of the Prophet (PBUH) released eight thousand slaves within one day. May Allah be pleased with all of them. (Ibn `Allan and Nuzhat Al-Muttaqin).