Here's the biblical reference:

Jesus Forbids Oaths

"Again you have heard that it was said to those of old "you shall not swaer falsely, but shall perform oaths to the Lord."

But I say unto you, do not swear at all: neither by heaven, for it is God's throne: nor by the earth, for it is His footstool; nor by Jerusalem, for it is the city of the great King.

Nor shall you swaer by your head, because you cannot make one hair white or black.

But let your "Yes" be "yes" and your "No" be "No".  For whatever is more than these is from the evil one.

Matthew, 5:33-37.

Interesting, huh?

~Shelagh