I think by employee referral amount, you mean an amount given to an employee from whose reference some other employee has joined. This is not part of the salary, but it is like a commission. If the same is not a payout that will occur regularly, then it is advisable not to mention it in payslips.
It does not mean that the commission is not part of remuneration or salary, but it is a part of the salary to be reflected in the payslips, but not in respect of all employees, but strictly in respect of those employees whose remuneration contains commission as a component. This is true in the case of salespersons whose duty is to sell, and the salary or remuneration shall contain an element called commission based on sales. However, the employee referral amount is paid not as part of the contract of employment but as part of a separate arrangement.