A teaching of Jesus' during The Sermon on the Mount.
I find this fascinating, both in the context of the time during Jesus' life, and during our own time. Jesus tells us never to swear a vow when promising something. He says never to swear by the the kingdom of heaven, nor by the earth not even by your own hand. Everything is sacred in God's eyes, don't presume to swear by anything he does. Presumably, then (as now), people would swear casually using God's name - something I find quite annoying.
The beginning of verse 37 "Just say 'yes' or 'no'" touches me. We shouldn't make extravagant promises, just say yes and get on with it. (now if only I could actually get on with stuff, I would be happy :)