It is to be understood that 15 centuries ago, the institution of slavery was common in Arabs and when Islam spread, Islam took measures to get rid of the institution of slavery in a gradual process. Quran is very clear on freeing slaves in the later revealed verses. In verse 12 and 13 of Chapter Balad, Allah says: “And what can make you know what is [breaking through] the difficult pass? It is the freeing of a slave.”