Legal Notice after 30 years of owning a land

Hi, My dad bought a land on 1987(From "Zahith") and did settlement on 2015 to me. Now a person send a legal notice that the land belongs to his dad and his sons has only the rights to own it. I took EC and found the following, 1964 - Sale deed From Krishna to Veni 1972 - Partition Deed in favour of Jagir Hussian 1985 - Sale deed From Mohamed to Zahith 1987 - Sale deed from Zahith to my dad Now the sons of Jagir Hussian sent a legal notice that they have the right to own and my dad fabricated some false document to transfer the land in his name. From 1987 my dad did agriculture and paid the tax too. On 1989 patta is transfered to my dad name. For the last 2 years the land is not cultivated I have a confusion, Since there is no sale deed or any document in favour of "Mohamed" in the EC, How "Mohamed" got patta.(From 1975 "Mohamed" name is in the patta) Please guide me to know how "Mohamed" got this land. And also the family of Jagir Hussian did not showed any objection when Mohamed sold the property to Zahith and Zahith to my dad