It  is  mandatory  to  quote  PAN  on  return  of  income,  all  correspondence  with  any income  tax  authority.  From  1  January  2005  it  will  be  mandatory  to  quote  PAN  on challans for any payments due to Income Tax Department.
It  is  also  compulsory  to  quote  PAN  in  all  documents  pertaining  to  the  following financial transactions :-

(a)  sale or purchase of any immovable property valued at five lakh rupees or more;
(b)  sale  or  purchase  of  a  motor  vehicle  or  vehicle, [the  sale  or  purchase  of  a  motor
vehicle or vehicle does not include two wheeled vehicles, inclusive of any detachable
side-car having an extra wheel, attached to the motor vehicle;]
(c)  a time deposit, exceeding fifty thousand rupees, with a banking company ;
(d)  a  deposit,  exceeding  fifty  thousand  rupees,  in  any  account  with  Post  Office
Savings Bank;