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"O Prophet (Muhammad sal-Allaahu 'alayhe wa sallam)! Verily, We have made lawful to you your wives, to whom you have paid their Mahr (bridal money given by the husband to his wife at the time of marriage), and those (captives or slaves) whom your right hand possesses - whom Allaah has given to you, and the daughters of your 'Amm (paternal uncles) and the daughters of your 'Ammah (paternal aunts) and the daughters of your Khal (maternal uncles) and the daughters of your Khalah (maternal aunts) who migrated (from Makkah) with you, and a believing woman if she offers herself to the Prophet, and the Prophet wishes to marry her; a privilege for you only, not for the (rest of) the believers, Indeed We know what We have enjoined upon them about their wives and those (captives or slaves) whom their right hands possess, - in order that there should be no difficulty on you, And Allaah is Ever Oft-Forgiving, Most Merciful." [Al-Ahzaab (33):50]

Narrated Talha bin 'Ubaidullah:

A man from Najd with unkempt hair came to Allah's Apostle and we heard his loud voice but could not understand what he was saying, till he came near and then we came to know that he was asking about Islam. Allah's Apostle said, "You have to offer prayers perfectly five times in a day and night (24 hours)." The man asked, "Is there any more (praying)?" Allah's Apostle replied, "No, but if you want to offer the Nawafil prayers (you can)." Allah's Apostle further said to him: "You have to observe fasts during the month of Ramad, an." The man asked, "Is there any more fasting?" Allah's Apostle replied, "No, but if you want to observe the Nawafil fasts (you can.)" Then Allah's Apostle further said to him, "You have to pay the Zakat (obligatory charity)." The man asked, "Is there any thing other than the Zakat for me to pay?" Allah's Apostle replied, "No, unless you want to give alms of your own." And then that man retreated saying, "By Allah! I will neither do less nor more than this." Allah's Apostle said, "If what he said is true, then he will be successful (i.e. he will be granted Paradise)." [The Book of Faith Volume 1, Book 2, Hadeeth 44]