The Prophet; said, "A slave (of Allah) may utter a word which pleases Allah without giving it much importance, and because of that Allah will raise him to degrees (of reward): a slave (of Allah) may utter a word (carelessly) which displeases Allah without thinking of its gravity and because of that he will be thrown into the Hell-Fire." (Using translation from Bukhārī 6478)
"إِنَّ الْعَبْدَ لَيَتَكَلَّمُ بالْكَلِمَة من رِضْوَان الله لَا يُلْقِى لَها بالًا يَرْفَعُهُ اللهُ بهَا دَرَجات، وَإنِّ الْعَبْدَ لَيَتَكلَّمُ بالْكَلِمَةِ مِنْ سُخطِ اللهِ لَا يُلقى لَها بَالًا يَهْوى بهَا في جَهَنَّم".