Recover tax paid on retention bonus after quitting the company

Hi

I was paid a retention bonus from my company in April, 2018 and I left the company in July where I had to return the bonus amount. Now on receipt of that bonus amount, I had paid tax. When returning that amount, I had to return that amount in full, so basically I have paid some excess tax so far. What documents do I need to procure from the company to claim that refund in my ITR for next year?

Thanks