What they want to see is incoming CASH FLOW. You just have to show them that you have set up a regular monthly distribution from your IRA/401K and enough money in the account for 36 months of that distribution amount. IOW, they want it to look like a paycheck.
It is all detailed out in the FNMA rules for lenders. Most mortgage people don't know about this, because 99% of borrowers have a paycheck and are not retired.
The underwriter who keyed me to this said "You can stop this after the mortgage closes." Just as you could lose your job after the mortgage closes.

I can corroborate RayVT's comment, and it's an important and useful point that retirees buying a house should be aware of:

In my situation, I had a very-clued in mortgage broker: I was willing to pay cash and stated that in the offer, but actually paying cash would have required cashing out some assets. He got me a very competitive loan in under 30 days from offer to close (part of the attractiveness to the seller was "all cash, quick close". ). Exactly as RayVT said, I had initiated monthly distributions from IRA when I knew I would be buying so the additional IRA distrib plus my retirement cash flow satisfied mortgage rules regarding a sufficient income. I stopped the distribution after closing. It's perfectly legal - as RayVT said, you might not have been retired but instead employed with a fat paycheck, approved for a loan, and then unfortunately lost your job after closing.
As far as using a mortgage broker: they are paid by tacking on a small percentage as a fee (you don't see this, you look at the net rate and APR). But if they get you a better deal than you would have found on your own, are extremely knowledgable and go above and beyond as in my case , then a net tiny fee for service, if it in facts net positive at all compared to what you could have done on your own, is well worth it.