كتاب الحج
15  
The Book of Pilgrimage
(88)
Chapter: Al-Madinah eliminates its Dross and it is also called Tabah, and Taibah
(88)
باب الْمَدِينَةِ تَنْفِي شِرَارَهَا ‏‏
Sahih Muslim 1382 a

Abu Huraira (Allah be pleased with him) reported Allah's Messenger (way peace be upon him) as saying:

I have been commanded (to migrate) to a town (Medina) which would overpower other towns. They (the people) call it Yathrib; its correct name is (in fact) Medina. It eliminates (bad) people just as a furnace removes the alloy of iron.
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكِ بْنِ أَنَسٍ، - فِيمَا قُرِئَ عَلَيْهِ - عَنْ يَحْيَى بْنِ، سَعِيدٍ قَالَ سَمِعْتُ أَبَا الْحُبَابِ، سَعِيدَ بْنَ يَسَارٍ يَقُولُ سَمِعْتُ أَبَا هُرَيْرَةَ، يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ أُمِرْتُ بِقَرْيَةٍ تَأْكُلُ الْقُرَى يَقُولُونَ يَثْرِبَ وَهْىَ الْمَدِينَةُ تَنْفِي النَّاسَ كَمَا يَنْفِي الْكِيرُ خَبَثَ الْحَدِيدِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1382a
In-book reference : Book 15, Hadith 557
USC-MSA web (English) reference : Book 7, Hadith 3189
  (deprecated numbering scheme)