So help you god

SO HELP YOU GOD. The formula at the end of a common oath, as administered to a witness who testifies in chief.

References in classic literature ?
'Take the book in your right hand this is your name and hand- writing you swear that the contents of this your affidavit are true so help you God a shilling you must get change I haven't got it.'
The House Committee on Natural Resources has in the past used a witness oath that reads, "Do you solemnly swear or affirm that the testimony that you are about to give is the truth, the whole truth, and nothing but the truth, so help you God?" Its proposed new version read, "Do you solemnly swear or affirm, under penalty of law, that the testimony that you are about to give is the truth, the whole truth, and nothing but the truth?"
If and when haled to court for demanding kickbacks, electioneering, rigging bids, tax evasion, whatever, he quickly answers "I do" and just as quickly forgets the question, "Do you solemnly swear that you will tell the truth, the whole truth, and nothing but the truth, so help you God?"
BEFORE YOU CAN TESTIFY HOW THE WORDS "UNDER GOD" IN THE PLEDGE IS AN UNCONSTITUTIONAL SANCTIONING OF RELIGION BT GOVERNMENT, PLEASE PLACE YOUR RIGHT HAND ON THIS BIBLE AND SWEAR THAT YOU'LL "TELL THE TRUTH, THE WHOLE TRUTH, AND NOTHING BUR THE TRUTH, SO HELP YOU GOD."
A review revealed that half the respondents wished to retain the traditional oath, in which witnesses are posed the question: "Do you swear to tell the truth, the whole truth and nothing but the truth, so help you God?" (National Post, June 10, 1999).