كتاب التوبة
50  
The Book of Repentance
(3)
Chapter: The Virtue Of Constant Dhikr, Thinking Of The Hereafter, And Remembering That Allah Is Always Watching; Permissibility Of Stopping That Sometimes, And Attending To Worldly Matters
(3)
بَاب فَضْلِ دَوَامِ الذِّكْرِ وَالْفِكْرِ فِي أُمُورِ الْآخِرَةِ وَالْمُرَاقَبَةِ وَجَوَازِ تَرْكِ ذَلِكَ فِي بَعْضِ الْأَوْقَاتِ وَالِاشْتِغَالِ بِالدُّنْيَا
Sahih Muslim 2750 c

Hanzala Taimi Ufayyidi, the scribe of Allah's Messenger (ﷺ), reported:

We were in the presence of Allah's Messenger (ﷺ) and he brought to our minds the problems pertaining to Paradise and Hell-Fire. The rest of the hadith is the same.
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا الْفَضْلُ بْنُ دُكَيْنٍ، حَدَّثَنَا سُفْيَانُ، عَنْ سَعِيدٍ الْجُرَيْرِيِّ، عَنْ أَبِي عُثْمَانَ النَّهْدِيِّ، عَنْ حَنْظَلَةَ التَّمِيمِيِّ الأُسَيِّدِيِّ الْكَاتِبِ، قَالَ كُنَّا عِنْدَ النَّبِيِّ صلى الله عليه وسلم فَذَكَّرَنَا الْجَنَّةَ وَالنَّارَ ‏.‏ فَذَكَرَ نَحْوَ حَدِيثِهِمَا ‏.‏
Reference : Sahih Muslim 2750c
In-book reference : Book 50, Hadith 16
USC-MSA web (English) reference : Book 37, Hadith 6625
  (deprecated numbering scheme)