One may utter a word thoughtlessly and, as a result of this, he will fall down into Hellfire for a distance greater than the distance between the east and the west
Authentic hadithعن أبي هريرة رضي الله عنه أَنَّهُ سَمِعَ النَّبيَّ -صلّى اللهُ عليه وسَلَّم يقول-: «إن العبد ليتكلم بالكلمة ما يتبين فيها يزلُّ بها إلى النار أبعدَ مما بين المشرق والمغرب».
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "One may utter a word thoughtlessly and, as a result of this, he will fall down into Hellfire for a distance greater than the distance between the east and the west."
Explanation
The Prophet (may Allah's peace and blessings be upon him) informs us that there are people who do not think before they speak whether what they are about to say is good or evil. The result of this thoughtlessness is that this person, if he said something that displeases Allah, would expose himself to the punishment of Allah in the fire of Hell. He may speak a word that is so evil that it would cause him to fall into Hellfire farther than the distance between the east and the west.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).