Sahih Muslim Book 37, Hadith Number 6625.

Chapter : The excellence of constant remembrance of Allah and meditation in the affairs of the hereafter and permission of abandoning it at times and attending to the affairs of the world.

Hanzala Taimi Ufayyidi, the scribe of Allah’s Messenger (may peace be upon him), reported: We were in the presence of Allah’s Messenger (may peace be upon him) and he brought to our minds the problems pertaining to Paradise and Hell-Fire. The rest of the hadith is the same.