Disclaimer: Your question does not have a country tag. However, from your name & from my experience, it look like India. If not, please let me know, I shall delete the answer.
The HR will ask you for your offer letter to verify 2 things:
- That you really work for the company
- You actually get the salary you mentioned
As paparazzi mentioned above, take your pay slips and your bank statement for 1.3 years.
In addition, since you have worked for 1.3 years, you would also have your govt. issued Form26AS from the tax portal, which mentions your net taxable income.
These 3 together will prove, beyond doubt that you work for the company and draw the salary you mentioned.
Also mention that you have not been issued with an offer letter from your company as yet.