Article 6, Clause 2

Document 29
John Breckenridge, Municipal Authority to Impose Taxes
28 Apr. 1806 1 Ops. Atty Gen. 157
1 Ops. Atty Gen. 157 
I am of opinion that there rests no power in the city council, nor in any department of the government of Orleans, to tax the property of the United States within that territory. I believe the exercise of such power has never been before attempted in any part of the United States, and I think the general government ought not to admit the principle. Laying the tax will be harmless, for I see no means by which the payment of it can be enforced.

The Founders' Constitution
Volume 4, Article 6, Clause 2, Document 29
http://press-pubs.uchicago.edu/founders/documents/a6_2s29.html
The University of Chicago Press