Voucher Mandatory?

Yes , maintaining voucher are mandatory , even if it is in low or not , because if you have supported documentary evidence then you can attached to the voucher , but in case of Cash payment voucher is must for example a office cleaning person , tea vendor for office , they are not issuing any Invoice , so you have to take signature of that person on the voucher for money receipt also preparing voucher the in important for Audit purpose in such case . So in my opinion in all cases Voucher is must .

Let me know which voucher ?

Is it related to GST or payment or receipt voucher ?

GST realted payment receipt voucher are mandatory as per the CGST act .