Words of the Buddha:
“And what is right speech? Abstaining from lying, from divisive speech, from abusive speech, & from idle chatter: This is called right speech.”
— SN 45.8
Speak only the speech
that neither torments self
nor does harm to others.
That speech is truly well spoken.
Speak only endearing speech,
speech that is welcomed.
Speech when it brings no evil
— Sn 3.3
In addition, the following quote establishes the relationship between upright speech and some other parts of the eight-fold path:
“And how is right view the forerunner? One discerns wrong speech as wrong speech, and right speech as right speech. And what is wrong speech? Lying, divisive tale-bearing, abusive speech, & idle chatter. This is wrong speech…
“One tries to abandon wrong speech & to enter into right speech: This is one’s right effort. One is mindful to abandon wrong speech & to enter & remain in right speech: This is one’s right mindfulness. Thus these three qualities — right view, right effort, & right mindfulness — run & circle around right speech.”
— MN 117
So, on a practical level, right view is knowing when you are speaking wisely or unwisely. Right effort is trying to speak with wisdom and without causing harm, and right mindfulness helps us remember (again and again) to try to abandon our careless ways and encourage our careful ways. These three aspects of the path come together when we practice cultivating skilful speaking.
All quotes are from: http://www.accesstoinsight.org/ptf/dhamma/sacca/sacca4/samma-vaca/