Slavery have been a part of islam from the start and sharia regulate this practice. Despite this some muslims claim that the quran is a perfect manual to achieve abolition. The number of slaves from arab slave trade are estimated to be 17 millions between the years 650-1905. Almost 3 million people are believed to be slaves in the middle east and north africa today.
From waiting for freedom to back into slavery at the hands of Muhammad:
Narrated Jabir bin Abdullah:
A man decided that a slave of his would be manumitted after his death and later on he was in need of money, so the Prophet took the slave and said, “Who will buy this slave from me?” Nu’aim bin ‘Abdullah bought him for such and such price and the Prophet gave him the slave.
Sahih Bukhari (41.598)
Sex with slaves are allowed, even if they are married:
Also (forbidden are) women already married, except those (captives and slaves) whom your right hands possess. Thus has Allah ordained for you. All others are lawful, provided you seek (them in marriage) with Mahr (bridal money given by the husband to his wife at the time of marriage) from your property, desiring chastity, not committing illegal sexual intercourse, so with those of whom you have enjoyed sexual relations, give them their Mahr as prescribed; but if after a Mahr is prescribed, you agree mutually (to give more), there is no sin on you. Surely, Allah is Ever All-Knowing, All-Wise.
Owners of female captives are worried about them getting pregnant while raped and that it will decrease the profit when they are sold. Muhammad only talks about how coitus innteruptus is wrong:
Narrated Abu Said Al-Khudri:
…that while he was sitting with Allah’s Apostle he said, “O Allah’s Apostle! We get female captives as our share of booty, and we are interested in their prices, what is your opinion about coitus interruptus?” The Prophet said, “Do you really do that? It is better for you not to do it. No soul that which Allah has destined to exist, but will surely come into existence.
Sahih Bukari (34.432)
Muhammed buy one arab slave at the cost of two black ones so blacks was seen as worth half of aothers. He didn’t free the bought slave afterwards:
Jabir (Allah be pleased with him) reported: There came a slave and pledg-ed allegiance to Allah’s Apostle (may peace be upon him) on migration; he (the Holy Prophet) did not know that he was a slave. Then there came his master and demanded him back, whereupon Allah’s Apostle (may peace be upon him) said: Sell him to me. And he bought him for two black slaves, and he did not afterwards take allegiance from anyone until he had asked him whether he was a slave (or a free man)
Sahih Muslim 3901