Sale of muslim ancestral land

I purchased agricultural land from a person that was his ancestral property. Now his son has filed a petition against his father claiming that he can't sell their ancestral land, though the regstration of the sale-deed has already been done in my favour. Am I legally safe? Seller & buyer, both are Muslims.

Further I want to ask how long does a curt take to decide such civil suits.

Warm Regards!