Received letter from IRS that I didn't report income from my home sale in 2017. I bought a new house with proceeds, which were less than $250k. How do I respond?

I received a letter from the IRS saying I didn't report my home sale income in 2017. I bought a new home immediately after the sale. I made less than $250k (filed jointly). What form do I need to send the IRS to show I didn't have income? They said I don't need to file an amended return. Thx!