I have to admit, I didn’t know where to look for this particular law myself. It’s not really part of the Constitution. It’s in the federal law books, which were written accumulatively down through the years by Congress. The following info is from Wikipedia, a good source.
Federal law originates with the Constitution, which gives Congress the power to enact statutes for certain limited purposes like regulating interstate commerce. The United States Code is the official compilation and codification of the general and permanent federal statutes.
United States Code
The law we are looking for, 1182 (f), is in this compilation of laws, in title 8. It took me a while to find it, because there are so many titles and chapters and subchapters and parts. But I finally found it. I had to click six different times and then at the end I had to wade through pages and pages of stuff to find (f). The following is that break down. So you go first to title 8, then to chapter 12, then to Subchapter II, then to section 1182, then you find f.
Title 8 of the United States Code – Aliens and Nationality
Chapter 12 – IMMIGRATION AND NATIONALITY
Subchapter II – IMMIGRATION (sections 1151 to 1381)
Part II – Admission Qualifications for Aliens; Travel Control of Citizens and Aliens (sections 1182 to 1189)
Section 1182 – Inadmissible Aliens
1182 (f) Suspension of entry or imposition of restrictions by President
Whenever the President finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may by proclamation, and for such period as he shall deem necessary, suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate. Whenever the Attorney General finds that a commercial airline has failed to comply with regulations of the Attorney General relating to requirements of airlines for the detection of fraudulent documents used by passengers traveling to the United States (including the training of personnel in such detection), the Attorney General may suspend the entry of some or all aliens transported to the United States by such airline.