Affair is said nowhere, this a TH summary is of an FT article about a close relationship (with extra clickbait). The source for both is an internal memo (anonymous sources) from a year ago

FT: https://archive.is/qu5eo

> US officials earlier this year wrote a memo, based on information obtained by American intelligence

NYT: https://archive.ph/K7bih

> Roughly a year ago, Mr. Trump was briefed about the intelligence findings, which have not been previously reported, and chose not to do anything with the information.