Barbara Levergood. Data Services Librarian, Bowdoin College is the author of a comprehensive, detailed and timely LibGuide resource – Congressional Documents: Impeachment – A selection of materials about and documenting the major events in the impeachment (House) and trial (Senate) of a U.S. President.  She calls out this specific reference – The oath before the trial of the impeachment of Clinton: Congressional Record (Bound Edition), Volume 145 (1999), Part 1, Senate, Page 274

…The PRESIDENT pro tempore. Will you place your left hand on the Bible, and raise your right hand. Do you solemnly swear that in all things appertaining to the trial of the impeachment of William Jefferson Clinton, President of the United
States, now pending, you will do impartial justice according to the Constitution and laws, so help you God?
The CHIEF JUSTICE. I do.
At this time I will administer the oath to all Senators in the Chamber in conformance with Article I, section 3, clause 6, of the Constitution and the Senate’s impeachment rules. Will all Senators now stand and raise your right hand.
Do you solemnly swear that in all things appertaining to the trial of the impeachment of William Jefferson Clinton, President of the United States, now pending, you will do impartial justice according to the Constitution and laws, so help you God?