The IRS works predominantly on the honor system until you get audited. They will want proof at that time. You will need to have proof that 1)You purchased it 2)You purchased it New 3)You put it to use 4) This one is not critical but, proof that it qualifies for the credit.
I would keep copies of supporting documents such as 1)Bill of Sale from the Dealer 2)Title 3)Insurance or Registration. Attach them to the file copy of your return and rest easy.