As the sale is taking place now you will have to show such income in the ITR next year i.e. while filing return of income for FY 2018-19. You have just received a payment and I am assuming you have not executed any sale deed.
Also since you are saying that it is your ancestral property and therefore I would suggest you to get it's market value as at FY 2001-2002 because I am assuming that I would have been purchased before 2001 and so for calculating indexed cost of acquisition you need to get it's fair market value for FY 2001-2002.
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