source document

My mother’s aunt (Khala), decided to sell her land & give 50% of the money to my mother to help us out. She sold her land to my mamu (ami’s Brother) and told her to give money directly to my mother. Now we want some kind of document for the bank as they want source document to confirm where this money came from. All the transactions were done through A/c payees cheque my mamu gave which was deposited to my amis a/c. Therefore I want to ask;
1.What type of document will be required?
2. What should be the contents
3. Should it be on stamp paper
4. Who will sign the document