Registering representative for MINOR on incometax portal

For registering a representative for a minor of e-filing portal of income tax website, 4 documents are required to be uploaded ....of these 3 documents are simple and clear ....minor's PAN card, parent's PAN card, minor's birth certificate....what I am unable to understand is that which document should be uploaded against "Documentary proof of income earned by the minor by doing manual work or by doing any activity involving application of his or her skill, talent or specialized knowledge and experience"...I uploaded bank passbook copy which was rejected...kindly guide me in this regard.