Returning Joining Bonus

My company paid some joining bonus to me which was paid in equal installments over 12 months after joining. There was no condition mentioned in my offer letter that I need to return the joining bonus in case I decide to leave within 12 months of joining. Now I have resigned the company after 8 months of joining and my company is asking me to return the 8 equal installments paid to me as joining bonus including tax.
Is this legally correct?? Can I dispute this decision ??

Does your company website or HR department have any such policy stipulation regarding terms and conditions of joining bonus?
Also check your offer letter-does it mention that management has mentioned that rules,procedures etc are liable to change at discretion of management.
Normally bonus is recovered if employee leaves within 12 months of joining.
However you can fight it out, if you have the time and energy since there are some looopholes like no mention about repayment etc in your case.
This will have tax implications also.