Not a word does he (or she) utter, but there is a watcher by him ready (to record it).
Narrated Abu Huraira:
The Prophet (sallallaahu alayhi wa sallam) 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."
[Sahiih al-Bukhaari Hadiith No. 6478]
Ibn al-Qayyim said:
Indeed, the slave (of Allaah) will come on the Day of Judgment with good deeds the like (in magnitude) of a mountain. He will find that his tongue has destroyed them all.
And he will come with bad deeds the like (in magnitude) of a mountain. He will find that his tongue has destroyed them all because of the abundance of dhikr of Allaah (which he used to make).