What is to be provided as proof of reference?

i have an interview next month. In application form , I was asked to give two references. I gave the references of two professors who are teaching me. But now i'm qualified for the interview, i've been asked to bring the PROOFS OF THE REFERENCES. What is to be presented as this proof? Does a copy of their ID cards r necessary or a detailed letter from each of them claimin to know me n my details is needed ? Or is it something else ?
PLEASE HELP ME ....