Sahih Bukhari Volume 9, Book 96, Hadith 7324

حَدَّثَنَا إِسْمَاعِيلُ، حَدَّثَنِي مَالِكٌ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ السَّلَمِيِّ، أَنَّ أَعْرَابِيًّا، بَايَعَ رَسُولَ اللَّهِ صلى الله عليه وسلم عَلَى الإِسْلاَمِ، فَأَصَابَ الأَعْرَابِيَّ وَعْكٌ بِالْمَدِينَةِ، فَجَاءَ الأَعْرَابِيُّ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ يَا رَسُولَ اللَّهِ أَقِلْنِي بَيْعَتِي‏.‏ فَأَبَى رَسُولُ اللَّهِ صلى الله عليه وسلم ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي‏.‏ فَأَبَى ثُمَّ جَاءَهُ فَقَالَ أَقِلْنِي بَيْعَتِي‏.‏ فَأَبَى فَخَرَجَ الأَعْرَابِيُّ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِنَّمَا الْمَدِينَةُ كَالْكِيرِ، تَنْفِي خَبَثَهَا، وَيَنْصَعُ طِيبُهَا ‏"‏‏

Narrated By Jabir bin 'Abdullah As-Salami : A bedouin gave the Pledge of allegiance for embracing Islam to Allah's Apostle, and then he got an attack of fever in Medina and came to Allah's Apostle: and said, "O Allah's Apostle! Cancel my pledge." Allah's Apostle refused to do so. The bedouin came to him again and said, "Cancel my pledge," but he refused again, and then again, the bedouin came to him and said, "Cancel my pledge," and Allah's Apostle refused. The bedouin finally went away, and Allah's Apostle said, "Medina is like a pair of bellows (furnace), it expels its impurities while it brightens and clears its good."