Article 2, Section 1, Clause 8
Document 1
Records of the Federal Convention
[2:146, 172; Committee of Detail, IV, IX]
9. and shall swear fidelity to the union, (as the legislature shall direct.) by taking an oath of office*
. . . . .
[Editors' note.--Words in parentheses were crossed out in the original.]
Before he shall enter on the Duties of his Department, he shall take the following Oath or Affirmation, "I ------ solemnly swear,--or affirm,--that I will faithfully execute the Office of President of the United States of America."
[2:427; Madison, 27 Aug.]
Col: Mason & Mr. Madison, moved to add to the oath to be taken by the supreme Executive "and will to the best of my judgment and power preserve protect and defend the Constitution of the U. S."
Mr. Wilson thought the general provision for oaths of office, in a subsequent place, rendered the amendment unnecessary--
On the question
N.H. ay--Mas--abst Ct ay--Pa ay. Del. no. Md. ay. Va. ay--N. C. abst S. C. ay. Geo. ay. [Ayes--7; noes--1; absent--2.]
[2:575, 599; Committee of Style]
Before he shall enter on the duties of his department, he shall take the following Oath or Affirmation, "I ------ solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States of America, and will to the best of my judgment and power, preserve, protect and defend the Constitution of the United States."
. . . . .
(g) Before he enter on the execution of his office, he shall take the following oath or affirmation: "I ------, do solemnly swear (or affirm) that I will faithfully execute the office of president of the United States, and will to the best of my judgment and power, preserve, protect and defend the constitution of the United States."
The Founders' Constitution
Volume 3, Article 2, Section 1, Clause 8, Document 1
http://press-pubs.uchicago.edu/founders/documents/a2_1_8s1.html
The University of Chicago Press
Farrand, Max, ed. The Records of the Federal Convention of 1787. Rev. ed. 4 vols. New Haven and London: Yale University Press, 1937.