Income tax on non proof income

Sir, I am the Broker of Land in Rural area. I am earning in cash 4.00 lakhs in one financial year from said land selling brokerage. But there is no provisions to making any papers in support of said income i.e. 4.00 lakhs. But I want to pay income tax on said 4.00 lakhs. This is possible ??? If yes, pls. clarify how to pay income tax on without any proof of income... thanks