take it to the bank 

Figurative: "What was said is the absolute truth and can be verified by a third party source." Comes from an obscure reference to cheques (or checks, if you're American), in that such is a guarantee that you can take the document to the bank and redeem it for its face value.

Note the pronoun 'it' can be varied accordingly.

"Mount Everest is the tallest geographical point on this planet, and you can take that to the bank."

take it to the bank by ke6isf October 25, 2003