Yes, you report it as a gift. If you later pay it back, you committed fraud.
[edit] to clarify what may be a point of confusion:
Rich parents are often in a position to comfortably gift their kid(s) tens of thousands of dollars for their first down payment.
Non-rich parents are more often in a position to somewhat-uncomfortably “gift” that money. It’s a gift on paper, but it’s actually a loan and they need the money back eventually (maybe for the next-oldest kid to borrow for the same purpose, lol). That’s when it’s fraud.
You agree it’s illegal (it is) but don’t think it can be proven?
“Here are bank records showing the defendant received $5,000 dollars. Here are further records that show payments back to the person who sent the original $5,000. They occur over a span of eleven months. Eight transactions are for $500, one is for $450, and a later one is for $550, totaling $5,000. Only one month in this otherwise contiguous span—December—is skipped, with no payment occurring. Mr. [defendant], was this $5,000 in fact a loan?”
[edit] to clarify what may be a point of confusion:
Rich parents are often in a position to comfortably gift their kid(s) tens of thousands of dollars for their first down payment.
Non-rich parents are more often in a position to somewhat-uncomfortably “gift” that money. It’s a gift on paper, but it’s actually a loan and they need the money back eventually (maybe for the next-oldest kid to borrow for the same purpose, lol). That’s when it’s fraud.