Why it is important to speak well and use suitable words?
It is narrated from Imam Reza (A.S) who said: One of the bases of wisdom is silence . One of the main bases of wisdom is to know when to speak, when to stay silent and speak less. Many of us need to learn the lesson of silence, how to speak, when to speak and for how long. When one decides to speak, should speak well and use suitable words because all of these (words) will be registered as described in Ayahs 10 and 11 of Surah Al-Infetar: “Verily over you are appointed angels to protect you; kind and honourable; writing down (your deeds and sayings).” The angels write down as you are dictating; therefore, be careful that you dictate well.
 Kafi, Vol. 2, Page 113.
Reference: Parts of speech by the Grand Ayatollah Javadi Amoli, Qom
The Trials of God
Beware of your thoughts and intentions
Why some wrong deeds look attractive to some people?
Bad deeds are the fuel for hellfire