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"O you who believe! When believing women come to you as emigrants, examine them, Allaah knows best as to their Faith, then if you ascertain that they are true believers, send them not back to the disbelievers, they are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them, But give the disbelievers that (amount of money) which they have spent [as their Mahr] to them, And there will be no sin on you to marry them if you have paid their Mahr to them, Likewise hold not the disbelieving women as wives, and ask for (the return of) that which you have spent (as Mahr) and let them (the disbelievers, etc.) ask back for that which they have spent, That is the Judgement of Allaah, He judges between you, And Allaah is All-Knowing, All-Wise." [Al-Mumtahanah (60):10]

Narrated Abu Burda's father:

Allah's Apostle said "Three persons will have a double reward:

1. A Person from the people of the scriptures who believed in his prophet (Jesus or Moses) and then believed in the Prophet Muhammad (i .e. has embraced Islam).
2. A slave who discharges his duties to Allah and his master.
3. A master of a woman-slave who teaches her good manners and educates her in the best possible way (the religion) and manumits her and then marries her."

[The Book of Knowledge Volume 1, Book 3, Hadeeth 97]