26 USC 6112 - Material advisors of reportable transactions must keep lists of advisees, etc.

(a) In general 
Each material advisor (as defined in section 6111) with respect to any reportable transaction (as defined in section 6707A (c)) shall (whether or not required to file a return under section 6111 with respect to such transaction) maintain (in such manner as the Secretary may by regulations prescribe) a list
(1) identifying each person with respect to whom such advisor acted as a material advisor with respect to such transaction, and
(2) containing such other information as the Secretary may by regulations require.
(b) Special rules 

(1) Availability for inspection; retention of information on list 
Any person who is required to maintain a list under subsection (a) (or was required to maintain a list under subsection (a) as in effect before the enactment of the American Jobs Creation Act of 2004)
(A) shall make such list available to the Secretary for inspection upon written request by the Secretary, and
(B) except as otherwise provided under regulations prescribed by the Secretary, shall retain any information which is required to be included on such list for 7 years.
(2) Lists which would be required to be maintained by 2 or more persons 
The Secretary may prescribe regulations which provide that, in cases in which 2 or more persons are required under subsection (a) to maintain the same list (or portion thereof), only 1 person shall be required to maintain such list (or portion).