Well, in Three Stories he said he thought he was coming down with a bit of the clap.

In his hallucination he lied by telling her that he wasn't experiencing any neurological symptoms.
In the real world? I'm searching the transcripts now.
Never mind - he lied at the Transplant Committee meeting in Control.
Cuddy: Dr. House, if you subvert or mislead this committee, you will be subject to disciplinary action.
House: Dr. Cuddy, do you have any reason to think that I would lie?
Cuddy: I simply want you to answer the question. Is there anything on the recipient exclusion criteria that would disqualify your patient from getting a heart?
House: No.