You may add the required line in the Agreement itself while engaging a contractor. You may refer to any book on Drafts and Pleading for the format of the required agreement. Alternatively, you may refer to the Labour Law Reporter, where all such drafts are published in monthly journals. You can also download it from any of the legal sites.
As mentioned by others in their views, the Principal Employer will always be responsible for the compliance of contractor employees, especially when the contractor does not have his own code numbers in EPF, ESI, and other statutory authorities.
Regards,
AK Chandok
RPFC (Retd.)
www.akchandok.com