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"And whoever of you have not the means wherewith to wed free, believing women, they may wed believing girls from among those (captives and slaves) whom your right hands possess, and Allaah has full knowledge about your Faith, you are one from another, Wed them with the permission of their own folk (guardians, Awliyaa' or masters) and give them their Mahr according to what is reasonable; they (the above said captive and slave-girls) should be chaste, not adulterous, nor taking boy-friends, And after they have been taken in wedlock, if they commit illegal sexual intercourse, their punishment is half that for free (unmarried) women, This is for him among you who is afraid of being harmed in his religion or in his body; but it is better for you that you practise self-restraint, and Allaah is Oft-Forgiving, Most Merciful." [An-Nisaa' (4):25]

Narrated Ibn 'Abbas:

Once I came riding a she-ass and had (just) attained the age of puberty. Allah's Apostle was offering the prayer at Mina. There was no wall in front of him and I passed in front of some of the row while they were offering their prayers. There I let the she-ass loose to graze and entered the row, and nobody objected to it. [The Book of Knowledge Volume 1, Book 3, Hadeeth 76]