One of my favorite Bible verses is this one, from the 5th chapter of the Gospel of Matthew:
But let your ‘Yes’ be ‘Yes,’ and your ‘No,’ ‘No.’ For whatever is more than these is from the evil one.
New King James Version
In the Authorized King James Version, which is my current Bible, it reads:
But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.