Is private investment return taxable?

Hi.

If I make some private investments, which do not have any formal proof unlike bank FD receipt and the privately made investment is also not under my pan. The returns I get, are they taxable?

The only record for example is some fund transfer from party A bank a/c to party B a/c and then back after some months. 

By private investment I mean a person (he may be from finance field) says give me say 1 lac and i will give you say 6% returns in 6 months and they do so in reality.

Thanks