...however there is no law that spells this out.
How does 26 USC 6012 sound?
Sec. 6012. - Persons required to make returns of income
(a) General rule
Returns with respect to income taxes under subtitle A shall be made by the following:
(1)
(A) Every individual having for the taxable year gross income which equals or exceeds the exemption amount, except that a return shall not be required of an individual -
(i) who is not married (determined by applying section 7703), is not a surviving spouse (as defined in section 2(a)), is not a head of a household (as defined in section 2(b)), and for the taxable year has gross income of less than the sum of the exemption amount plus the basic standard deduction applicable to such an individual,
(ii) who is a head of a household (as so defined) and for the taxable year has gross income of less than the sum of the exemption amount plus the basic standard deduction applicable to such an individual,
(iii) who is a surviving spouse (as so defined) and for the taxable year has gross income of less than the sum of the exemption amount plus the basic standard deduction applicable to such an individual, or
(iv) who is entitled to make a joint return and whose gross income, when combined with the gross income of his spouse, is, for the taxable year, less than the sum of twice the exemption amount plus the basic standard deduction applicable to a joint return, but only if such individual and his spouse, at the close of the taxable year, had the same household as their home.
Clause (iv) shall not apply if for the taxable year such spouse makes a separate return or any other taxpayer is entitled to an exemption for such spouse under section 151(c).