Accustom not your mouth to swearing; neither use yourself to the naming of the Holy One.
Read Chapter 23
Leander of Seville
Seek with the same scrupulousness to never offer oaths but to always speak the truth. As oaths are permitted to worldly people out of fear of deception, so for one leading a religious life oaths are prohibited, even when he has a clear conscience. The Lord says, "Let your speech be yes, yes, and no, no. Anything more comes from the evil one." Do not swear even when you speak the truth, because that comes from evil. It is said that it comes from evil because the need to swear comes from a bad conscience: one constrains a person to swear when he doubts his sincerity. You who should have the simplicity of the heart on your lips, why would you spontaneously bind yourself with an oath? Do not make a habit of either swearing in the good or perjuring in the bad. Speak the truth with all your heart, and there will be no need for oaths, as we read: "Do not accustom yourself to oaths, because by swearing many have fallen." - "Letter to His Sister Florentine 29"