Sale of Muslim Ancestral Property

I purchased agricultural land from a person that was his ancestral property. Now his son has filed a petition against his father (I am also respondent) claiming that he can't sell their ancestral land, though the registration 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 court take to decide such civil suits. Warm Regards!