The most important document in the GST system is the tax invoice. A correctly issued tax invoice will contain certain information such as details of registered person and supply, the GST rate and the amount of GST payable.
A tax invoice is an essential evidence to:
(a) determine the period when a supplier should include his taxable supplies and account for output tax
(c) determine the period when a purchaser should include his taxable purchases and support his input tax claim
The tax invoices must be kept for at least five years (seven years in Malaysia). It is not required to submit these tax invoices with your GST returns.