The form 1040 IS YOUR RETURN you filed ... save a PDF and print off the form they are asking for and comply with their request BUT only if you are SURE this is not a scam/phishing letter.
To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)