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"Verily, those who give Bai'a (pledge) to you (O Muhammad sal-Allaahu 'alayhe wa sallam) they are giving Bai'a (pledge) to Allaah, The Hand of Allaah is over their hands, Then whosoever breaks his pledge, breaks only to his own harm, and whosoever fulfills what he has covenanted with Allaah, He will bestow on him a great reward." [Al-Fath (48):10]

Narrated Sa'd:

Allah's Apostle distributed (Zakat) amongst (a group of) people while I was sitting there but Allah's Apostle left a man whom I thought the best of the lot. I asked, "O Allah's Apostle! Why have you left that person? By Allah I regard him as a faithful believer." The Prophet commented: "Or merely a Muslim." I remained quiet for a while, but could not help repeating my question because of what I knew about him. And then asked Allah's Apostle, "Why have you left so and so? By Allah! He is a faithful believer." The Prophet again said, "Or merely a Muslim." And I could not help repeating my question because of what I knew about him. Then the Prophet said, "O Sa'd! I give to a person while another is dearer to me, for fear that he might be thrown on his face in the Fire by Allah." [The Book of Faith Volume 1, Book 2, Hadeeth 26]