I kid you not

Earliest known use is from November 3, 1943 on The Centenary Conglomerate. A print reference of the phrase appeared in 1948 in the International Stereotypers' and Electrotypers' Union Journal, Volume 43: Boy!, Oh Boy! I said I asked for a headache when I volunteered for this job, and I kid you not when I repeat it. It next appeared as a phrase spoken by Lieutenant Commander Philip Francis Queeg in Herman Wouk's 1951 Pulitzer Prize-winning novel, "The Caine Mutiny": That's the Navy for you. Pass the buck and get a receipt. Act at discretion, hey? Well that's exactly what I'm going to do, and I kid you not. It was later popularized by Jack Paar, host of The Tonight Show from 1957 to 1962, who used this as his signature phrase. It humorously combines the colloquial verb kid with the archaic negation through not without do-support. Compare believe you me.

来源：wiktionary