(21) Whoever hates to revert to Kufr (atheism or disbelief) as he hates to be thrown in fire, is a part of faith

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا شُعْبَةُ، ‏‏‏‏‏‏عَنْ قَتَادَةَ، ‏‏‏‏‏‏عَنْ أَنَسٍ، ‏‏‏‏‏‏عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ "ثَلَاثٌ مَنْ كُنَّ فِيهِ وَجَدَ حَلَاوَةَ الْإِيمَانِ، ‏‏‏‏‏‏مَنْ كَانَ اللَّهُ وَرَسُولُهُ أَحَبَّ إِلَيْهِ مِمَّا سِوَاهُمَا، ‏‏‏‏‏‏وَمَنْ أَحَبَّ عَبْدًا لَا يُحِبُّهُ إِلَّا لِلَّهِ، ‏‏‏‏‏‏وَمَنْ يَكْرَهُ أَنْ يَعُودَ فِي الْكُفْرِ بَعْدَ إِذْ أَنْقَذَهُ اللَّهُ كَمَا يَكْرَهُ أَنْ يُلْقَى فِي النَّارِ".

Sahih hadith: Narrated Anas: The Prophet said, Whoever possesses the following three qualities will taste the sweetness of faith: 1. The one to whom Allah and His Apostle become dearer than anything else. 2. Who loves a person and he loves him only for Allah's sake. 3. Who hates to revert to disbelief (Atheism) after Allah has brought (saved) him out from it, as he hates to be thrown in fire.

Reference: Sahih al-Bukhari 1: Chapter 2, Hadith 21.