On the eve of Diwali, Ravi purchased two kilograms of sweets from Nandan Sweets. On consumption of sweets, his wife fell sick and was to be hospitalised. Ravi wanted to file a case in the consumer forum but could not do so because he did not have any proof of buying the sweets from Nandan sweets. Name the document that Ravi could have obtained for filling the complaint in the consumer forum.