That he heard Messenger of Allah ﷺ 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." (Using translation from Bukhārī 6477)
"إِنَّ الْعَبْدَ لَيَتَكَلَّمُ بالْكَلِمَةِ مَا يُتَبِّنُ فِيهَا، يَزلُّ بهَا في النَّار أَبْعَدَ مَا بَينَ الْمَشْرق، والْمَغْرب ".