Mu'adh ibn Jabal Radiyallahu 'anhu narrates that I asked: O Rasulallah!
Is all that we say recorded for us? (and will we be punished for it) He said: May your mother be bereaved of you (understand well) that there is nothing more that throws people over their noses into Hell-Fire than the harvest of their tongues. Indeed you remain safe as long as you are silent but when you speak it will either be recorded in your favour (a good deed) or against you (a sin). (Tabrani, Majma-'uz-Zawaid)
Note: "May your mother be bereaved of you" is, according to the Arab tradition, an address of affection and not of cursing.
'Abdullah Radiyallahu 'anhu narrates that he heard Rasullullah Sallallahu 'alaihi wasallam saying: Most of the faults that the son of Adam commits are by his tongue. (Tabrani, Majma-'uz-Zawaid)
A maid-slave of the daughter of Abul Hakam Radiyallahu 'anha narrates that she heard Rasullullah Sallallahu 'alaihi wasallam saying: Indeed a person comes so close to Paradise that between him and Paradise there remains a distance of an arm's length; then he speaks such a word because of which he is removed from Paradise more than the distance from Madinah to Sana'a.
(Musnad Ahmad, Majma-'uz- Zawaid)