The Treaty of Tripoli (1796)

(thanks to Geoffrey R. Stone for pointing this out!)

The Treaty of Tripoli
Article XI
As the Government of the United States of America is not, in any sense, founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquillity, of Mussulmen; and, as the said States never entered into any war, or act of hostility against any Mahometan nation, it is declared by the parties, that no pretext arising from religious opinions, shall ever produce an interruption of the harmony existing between the two countries.
[emphasis added]

From Wikipedia entry:
Official records show that after President John Adams sent the treaty to the Senate for ratification in May 1797, the entire treaty was read aloud on the Senate floor, including the famous words in Article 11, and copies were printed for every Senator. A committee considered the treaty and recommended ratification, and the treaty was ratified by a unanimous vote of all 23 Senators. It was the 339th time a recorded vote was taken in the Senate and only the third time a unanimous result was obtained. The treaty was reprinted in full in three newspapers, two in Philadelphia and one in New York City. There is no record of any public outcry or complaint in subsequent editions of the papers.
[emphasis added]

Gentle reminder:
U.S. Constitution
Article VI
This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrary notwithstanding.
[emphasis added]