Sahih al-Bukhari 29:17
Narrated Jabir:
A bedouin came to the Prophet (ﷺ) and gave a pledge of allegiance for embracing Islam. The next day he came with fever and said (to the Prophet (ﷺ) ), "Please cancel my pledge (of embracing Islam and of emigrating to Medina)." The Prophet (ﷺ) refused (that request) three times and said, "Medina is like a furnace, it expels out the impurities (bad persons) and selects the good ones and makes them perfect."
Reference
Sahih al-Bukhari 29:17
In-book reference
Book 29, Hadith 17
Arabic Text
حَدَّثَنَا عَمْرُو بْنُ عَبَّاسٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ، حَدَّثَنَا سُفْيَانُ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، عَنْ جَابِرٍ ـ رضى الله عنه ـ جَاءَ أَعْرَابِيٌّ النَّبِيَّ صلى الله عليه وسلم فَبَايَعَهُ عَلَى الإِسْلاَمِ، فَجَاءَ مِنَ الْغَدِ مَحْمُومًا، فَقَالَ أَقِلْنِي، فَأَبَى ثَلاَثَ مِرَارٍ، فَقَالَ " الْمَدِينَةُ كَالْكِيرِ، تَنْفِي خَبَثَهَا، وَيَنْصَعُ طَيِّبُهَا ".