Volume 8 · Book 76 · Hadith 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."
— Sahih Bukhari, Volume 8, Book 76: To make the Heart Tender (Ar-Riqaq), Hadith #484