Lets say there is a person, who is speaking/writing about a fact which according to him is true..

As soon as he finishes his speech/passage, another person wants to ask him "how do you know it (this fact) ?

Would it be correct for him to say {chay' DaSov ?} And by the use of the prefix {Da-} the {Sov} having as an elided object the entire preceding speech/passage ?