In German I have read an article and watched a documentary about it, but the best source was the book "Al Muqaddimah" by the brilliant 14th century Tunisian historian Ibn Khaldun, were the importance of the slaves is mentioned in detail. But it's a very thick book and I read it many years ago, so I can't give you a quote.
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One thing I have noticed: Normally, for Pakistanis "Islamic solidarity" is more important than for all other Muslims. Pakistanis are fanatically obsessed with the concept of Ummah! And when other Muslims talk about racial pride, Pakistanis become very sad, they are like "No, only Islam, no nations, no borders, we Muslims, no races, Islamic solidarity, only Ummah Ummah Ummah"......but at the moment when the black skin enters the arena, then the same Pakistanis become the most race-obsessed people on earth! Suddenly "We Muslims" doesn't exist anymore.
Pakistanis want everyone to care about Kashmir, but they themselves have absolutely no compassion towards Blacks!
Pakistanis are more racist towards Blacks than even Arabs, it's just pure hate, and not because of the behavior of Blacks, but only because of their skin color. And being Muslim doesn't shield Blacks from this hate in any way!
If there weren't any consequences that would follow, then Pakistanis would love to drop their nuclear weapons on Africa and clean the surface of earth from this black dirt!!!
But I don't want to put the blame only on Pakistanis, everyone is racist towards Blacks. This racism is epidemic. Muslims always talk about solidarity towards Palestine, and Kashmir, and Rohingas, and this and that, Ummah here and there, but this pure bullshit, it's mere lip-service…..
Anti-Black racism = that's the only unity that exists in the Muslim world!!!
And this once again shows that at least 90% of all non-black Muslims of the world are deep in their hearts atheists and do not believe in Islam, because they are obviously disgusted of Gods creation!!!
You are either a Muslim or a Black-hater, the one thing totally contradicts the other, you cannot be both, it's totally impossible!!!
So when in future someone claims to be a believer, you just have to ask "Would you accept that your daughter marries a Black man with a Black skin?"....If he hesitates for just a moment to answer, then you know that he is nothing but a pork-meat-eating infidel!!!