That we must not swear
In Solomon: “A man that swears much shall be filled with iniquity, and the plague shall not depart from his house; and if he swear vainly, he shall not be justified.” Of this same matter, according to Matthew: “(Again, you have heard that it was said to them of old, You shall not swear falsely, but shall perform unto the Lord your oaths.) I say unto you, Swear not at all: (neither by heaven, because it is God's throne; nor by the earth, because it is His footstool; nor by Jerusalem, because it is the city of the great King; neither shall you swear by your head, because you can not make one hair white or black.) But let your discourse be, Yea, yea; Nay, nay: (for whatever is fuller than these is of evil.”) Of this same thing in Exodus: “You shall not take the name of the Lord your God in vain.”
Source: The Treatises of Cyprian (New Advent)