The Messenger of Allah (SA) was always concerned about the wellbeing of the slaves, and he would always mention them at the times when he expected the greatest attention from his audience, such as the time of his death and
during the Farewell Pilgrimage, where he had the largest audience in his lifetime. “As for your slaves, male and female,” he exhorted them during the Farewell Pilgrimage, “feed them with what you eat yourself and clothe them with what you wear. If you cannot keep them or they commit any fault, discharge them. They are God’s people like unto you and be kind unto them.” [Gordon, M. (1989). Slavery in the Arab world. New York: New Amsterdam, p. 19.]
No other nation or religious group in the world treated slaves better than the Muslims did, as demonstrated by the aforementioned examples of instructions from Allah and His Messenger (SA). The following are the testimonies of non-Muslim historians and leaders regarding this very fact:
On the attitude of the Muslim master towards his slaves, American historian and philosopher Will Durant writes:
…he handled them with a genial humanity that made their lot no worse - perhaps better, as more secure - than that of a factory worker in nineteenth-century Europe… It is astonishing how many sons of slaves rose to high place in the intellectual and political world of Islam, how many, like Mahmud and the early Mameluks, became kings. [Will Durant, The Story of Civilization: Vol. 4, The Age of Faith (New York: Simon and Schuster, 1950), 209.]
At the end of the eighteenth century, Mouradgea d'Ohsson (an Armenian historian and diplomat who wrote extensively about the Ottoman Empire) declared:
There is perhaps no nation where the captives, the slaves, the very toilers in the galleys are better provided for or treated with more kindness than among the Muhammedans. [H. A. R. Gibb and J. H. Kramers, ed., The Encyclopaedia of Islam: New Edition, vol. 1 (Leiden: E. J. Brill, 1986), 35.]
Napoleon Bonaparte, a military leader who was Emperor of France from 1804 to 1814, is recorded as saying about the condition of slaves in Muslim countries:
In the East, slavery never had the same characteristics as in the West. The slavery of the East is that which is seen in the Holy Scripture: the slave inherits from his master and marries his daughter. Most of the Pashas had been slaves; a great number of grand viziers, all the Mamluks, Ali Bey al-Kabir, and Murad Bey had all been slaves. They started by performing the most menial services in the house of their master and subsequently rose in status because of their merit or through favours. In the West, on the other hand, the slave was always below the domestic servant; he occupied the lowest rank... [Christian Cherfils, Bonaparte et l'Islam d'après les Documents Français & Arabes (Paris: Libraire de la Cour d'Appel et de l'Ordre des Avocats, 1914), 124.]

With regard to the question of concubines and having sex with the slave girls, we must first say that the Prophet (SA) encouraged the masters to free their female slaves and marry them. He said: