Provided, however, that the Commissioner, in his discretion, may prescribe rules waiving such requirements in circumstances where he determines it is impracticable for such documentary evidence to be required. Ordinarily, documentary evidence will be considered adequate to support an expenditure if it includes sufficient information to establish the amount, date, place, and the essential character of the expenditure. For example, a hotel receipt is sufficient to support expenditures for business travel if it contains the following: name, location, date, and separate amounts for charges such as for lodging, meals, and telephone. Similarly, a restaurant receipt is sufficient to support an expenditure for a business meal if it contains the following: name and location of the restaurant, the date and amount of the expenditure, and, if a charge is made for an item other than meals and beverages, an indication that such is the case. A document may be indicative of only one (or part of one) element of an expenditure. Thus, a cancelled check, together with a bill from the payee, ordinarily would establish the element of cost. In contrast, a cancelled check drawn payable to a named payee would not by itself support a business expenditure without other evidence showing that the check was used for a certain business purpose.
If such element is the description of a gift, or the cost, time, place, or date of an expenditure, the corroborative evidence shall be direct evidence, such as a statement in writing or the oral testimony of persons entertained or other witness setting forth detailed information about such element, or the documentary evidence described in subparagraph (2) of this paragraph. If such element is either the business relationship to the taxpayer of persons entertained or the business purpose of an expenditure, the corroborative evidence may be circumstantial evidence.
If a taxpayer fails to maintain adequate records concerning a facility which is likely to serve the personal purposes of the taxpayer, it shall be presumed that the use of such facility was primarily personal.
In addition, he must maintain such records and supporting evidence as will substantiate each element of an expenditure (described in paragraph (b) of this section) in accordance with paragraph (c) of this section.
26 C.F.R. §1.274-5A