Right Speech, or Noble Speech

From the Abhaya Sutta:

In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial, unendearing and disagreeable to others, he does not say them.
In the case of words that the Tathagata knows to be factual, true, yet unbeneficial, unendearing and disagreeable to others, he does not say them.
In the case of words that the Tathagata knows to be factual, true, beneficial, yet unendearing and disagreeable to others, he has a sense of the proper time for saying them.
In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial, yet endearing and agreeable to others, he does not say them.
In the case of words that the Tathagata knows to be factual, true, but unbeneficial, yet endearing and agreeable to others, he does not say them.
In the case of words that the Tathagata knows to be factual, true, beneficial, and endearing and agreeable to others, he has a sense of the proper time for saying them. Why is that? Because the Tathagata has sympathy for living beings.

In every case, if it is not true, beneficial nor timely, one is not to say it. The Buddha followed this, for example, when asked questions of a purely metaphysical nature, unrelated to the goal, path or discipline that he taught. When asked a question such as "Is the universe eternal?", the Buddha dismissed the topic with the response: "It does not further." (or: "The personal possibilities (goals) assigned you are not furthered by an answer to an ultimate question about the universe's fate.")

This text was copied from Wikipedia, "Noble Eightfold Path"