My company is not paying BONUS in the name of non-existing policy doc

Hello, I have joined the company that I'm currently working in, 3 years back. My annual salary has been revised last year in March, 2016. They had included a performance bonus component in the salary of 1 lac rupees. I'm leaving my job on January 31st, 2017. As part of the final settlement, my company is denying me to pay my bonus on a pro-rata basis saying that it is not part of the "policy" of the company to give bonus to someone who is quitting before 1 year completion of their salary cycle. However, there is NO policy document that has been shared with me that lists the above clause. There is no mention of this even in my employement contract. As far as I know, a written policy document about the BONUS agreement does not exist, nor has it been shared with me. I do not know what I should be doing in this case. I feel helpless because it is my hard-earned money and I deserve the bonus. Kindly help me how to deal with this. Is there something that I could do legally?