I kid you not
I'm being completely serious right now; I am not kidding. A: "What? No way!" B: "Yes, I kid you not: Brad got the lead in the school play."
I kid you not.
Inf. I am not kidding you.; I am not trying to fool you. Bill: Whose car is this? Sally: It's mine. It really is. I kid you not. "I kid you not," said Tom, glowing. "I outran the whole lot of them."
I kid you not
sent. I am not kidding. (Attributed to the entertainer Jack Paar.) She is a great singer. I kid you not.