Islam said so.
Jizyah or jizya (جزْي
is the extra tax imposed on non-Muslims (Dhimmis) who live under Muslim rule according to the Qur'an and hadith:
Fight those who believe not in Allah nor the Last Day, nor hold forbidden that which hath been forbidden by Allah and His Messenger, nor acknowledge the religion of Truth, (even if they are) of the People of the Book, until they pay the Jizyah with willing submission, and feel themselves subdued.
Qur'an 9:29
"I call you to God and to Islam. If you respond to the call, then you are Muslims: You obtain the benefits they enjoy and take up the responsibilities they bear. If you refuse, then you must pay the jizyah. If you refuse the jizyah, I will bring against you tribes of people who are more eager for death than you are for life. We will then fight you until God decides between us and you." (Al Tabari, Volume XI)
Khalid bin Al-Waheed (Muslim General, 632AD)
Summon the people to God; those who respond to your call, accept it from them, but those who refuse must pay the poll tax out of humiliation and lowliness. If they refuse this, it is the sword without leniency. Fear God with regard to what you have been entrusted. (Al Tabari, Volume XII)
Umar ibn al-Khattab during the conquest of al-Basrah (636 CE)
Additional quotes can be found at Qur'an, Hadith and Scholars:Jizyah.
Once a land is conquered by Islamic armies the ruler can impose a taxation on those non-Muslims who will not convert to Islam.
Jizyah is paid as a sign of submission and gives Dhimmis some legal protection in return. Under dhimmitude (the status that Islamic law, the Sharia, mandates for non-Muslims) Dhimmis usually are not allowed to carry arms to protect themselves, serve in the army or government, display symbols of their faith, build or repair places of worship etc. If the conquered do not wish to pay or convert, their fate may very well be slavery (under which, rape is permitted) or (as evidenced in the quotes above) death.
The amount of the Jizyah tax and the way it was collected varied from time to time and from place to place, but when imposed, the forced payment of Jizyah greatly stimulated the conversion of non-Muslims into Islam.[1] In some cases the taxation of the non-Muslims was so profitable that the Islamic rulers prohibited their subjects from converting to Islam, lest they should lose their income.
http://wikiislam.net/wiki/Jizyah_(Tax)
I see only Islamic sources sited.
I do not cover up Christianity's sins like the crusades or the inquisition because it makes it worse and is not true. Why are you doing so for Islam?