26 USC 7501 - Liability for taxes withheld or collected

(a) General rule 
Whenever any person is required to collect or withhold any internal revenue tax from any other person and to pay over such tax to the United States, the amount of tax so collected or withheld shall be held to be a special fund in trust for the United States. The amount of such fund shall be assessed, collected, and paid in the same manner and subject to the same provisions and limitations (including penalties) as are applicable with respect to the taxes from which such fund arose.
(b) Penalties 
For penalties applicable to violations of this section, see sections 6672 and 7202.