Property in Dispute

Property Dispute. Hi, My grand father bought property of property of about 10 Acres from a Person 'X' in 1952 and registered in his name. My grandfather had 2 brothers. After couple of years, my grandfather and his brothers partitioned the property along with the other property in the year 1960. My father was the only son to my Grandfather. My father and Grandfather was doing agriculture till now from 1952 in the same land and we have all documents , tax paid receipts etc. Now, after partition the between the my grand father and his brothers the property finally ended in my hands from 2005. Now problem starts, we dont know who sold the property, date of sale Etc,Etc to mr.X. but Mr 'X' sold the property to my grandfather. Some unknown person mr 'Y' has filed a case against me saying property the property of 10Acres belongs him and mr ' X' is unknown, with out the knowledge Mr'Y', Mr 'X' has sold the 10 Acres to us. The person who has filed the case is grandson of Mr. 'Y'. The only proof he has submitted to Honourable court is mr. 'Y's in some documents earlier to 1930. I tried to find the sale deed from mr. 'Y' to mr 'X' in all the possible ways in all offices but no good. With this situation, i am helpless how to proceed and what basis. Could any one of you please suggest. Thanks in Advance Mr. Kumar S