One careless word
إِنَّ الْعَبْدَ لَيَتَكَلَّمُ بِالْكَلِمَةِ مِنْ رِضْوَانِ اللَّهِ لَا يُلْقِي لَهَا بَالًا، يَرْفَعُ اللَّهُ بِهَا دَرَجَاتٍ، وَإِنَّ الْعَبْدَ لَيَتَكَلَّمُ بِالْكَلِمَةِ مِنْ سَخَطِ اللَّهِ لَا يُلْقِي لَهَا بَالًا يَهْوِي بِهَا فِي جَهَنَّمَ
Inna-l-'abda la-yatakallamu bi-l-kalimati min ridwani-llahi la yulqi laha balan, yarfa'u-llahu biha darajat, wa inna-l-'abda la-yatakallamu bi-l-kalimati min sakhati-llahi la yulqi laha balan yahwi biha fi jahannam.
"A servant may say a word that pleases Allah without giving it much thought, and Allah raises him by degrees because of it. And a servant may say a word that angers Allah without giving it much thought, and it sinks him into Hell."
Narrated by Abu Hurayrah. Sahih al-Bukhari 6478
A short reflection
Words weigh more than they feel. Before the next easy remark, ask which of the two kinds it is.