You don't need a reference. The 401(k) loan is not secured by real estate of any kind, specifically it's not secured by the rental property it was used to purchase. If you don't pay back the 401(k) loan, then the 401(k) money is "garnished" to back back that loan - not the real estate. Therefore, the 401(k) loan interest is just flat out not deductible. Period.
