Every state and county officer, who is authorized by law to incur official expenses which are to be paid by the state or any county or out of any public funds of any character, is directed and required to make out accurate, itemized statements of such expenses, showing the date and amount of each separate item, and the purpose for which it was expended. The correctness of such expense account and the fact that it was actually incurred in the performance of official duty must be sworn to by such official before some officer qualified to administer oaths. In every instance where it is practicable to obtain a voucher representing the expense incurred, except for railway or pullman fare, such official shall obtain a voucher and attach same to the expense account. Such vouchers shall be numbered and referred to by number.