That was in his speech at Gettysburg on his 100 day action list.
Contract with the American Voter
#3 item in the first category.
* THIRD, a requirement that for every new federal regulation, two existing regulations must be eliminated;
https://assets.donaldjtrump.com/_landings/contract/O-TRU-102316-Contractv02.pdf
Goodness, FReepers are so amazing! I was wondering if I had dreamed Trump saying that, and you all came up with links to that statement. Thank you!