Speaking falsehood is sinful. Does it mean speaking is the truth is no sinful?
This page has extracts from a dozen suttas, which explain what "Right Speech" is.

Being "truthful" is an aspect, but  not the only aspect, of Right Speech: for example speech should also be affectionate and spoken with a mind of good-will, beneficial, and spoken at the right time. Also polite, not divisive, and so on.