I (do) declare!
Inf. I am surprised to hear that! (Old-fashioned.) Mary: I'm the new president of my sorority! Grandmother: I declare! That's very nice. A plane had landed right in the middle of the cornfield. The old farmer shook his head in disbelief. "I do declare!" he said over and over as he walked toward the plane.
I declare (to goodness)!
Rur. What a surprise! I declare to goodness! You certainly have grown since I saw you. Tom: Jim and Sally are going to get married. Mary: Well, I declare!