Zechariah 8:16: Speak Truth to Your Neighbor
Zechariah 8:16 does not ask God's people to talk honestly so that he will decide to bless them. It tells them to speak truth because he has already decided to do good, and their speech is meant to match his kindness.
Let Your Yes Be Yes: Matthew 5:33–37
Jesus does not merely tighten the rules about oaths. He calls his disciples to speech so plain and reliable that no oath is needed to back it up, and he grounds that honesty in who God is.