1) Tell your parents to transfer the amount to your bank account.
2) Write a gift deed by stating the money they are giving you, they won't take it back.
3) From where they got the amount? It can be written in the gift deed stamp paper too. if not,
4) then tell them to write a sworn affidavit stating their source of income.
I think these should do. Keep it simple. Don't write an affidavit which is 4-5 pages long. Just show the proof that after giving the money, still they have the capability to sustain. I hope this helps!