John Lightfoot on Matthew 5:34

[Swear not at all.] In the tract Demai are some rules prescribed to a religious man: among others, That he be not too much in swearing and laughing. Where the Gloss of R. Solomon is this; "means this, Be not much in oaths, although one should swear concerning things that are true: for in much swearing it is impossible not to profane." Our Saviour, with good reason, binds his followers with a straiter bond, permitting no place at all for a voluntary and arbitrary oath. The sense of these words goes in the middle way, between the Jew, who allowed some place for an arbitrary oath; and the Anabaptist, who allows none for a necessary one.