Sahih Bukhari
To make the Heart Tender (Ar-Riqaq)
Volume: 008 / Book Number: 076 / Hadith Number: 484
Narrated Abu Huraira:
That he heard Allah's Apostle saying, "A slave of Allah may utter a word without thinking whether it is right or wrong, he may slip down in the Fire as far away a distance equal to that between the east."