Documents from my current employer for H1b transfer while I m in India

Hi, I have US H1B visa from employer A and I traveled to USA on same visa for 4 months. Currently I am working in India with employer B. And A have revoked my petition. Now A wants to rehire me and ready for H1B transfer(CAT exempt). Now what are the documents require for H1B transfer from employer B with whom I am currently working.