Matthew 5:36
And do not take an oath by your head, for you cannot make one hair white or black. The final oath-form addressed is the personal oath — swearing by one's own head. The prohibition is grounded in the observation that one does not control one's own head: you cannot make one hair white or black. The logic is the same as for the cosmic oaths: swearing by anything that is ultimately God's possession is swearing by God, and the oath therefore should be replaced by simple honest speech.