Employment/reference letter from your manager/ HR is what you need. It should include your job title, work period, payment, work hours and duties.
I also included my job offer letter, which I don't think was mandatory.
If your reference letter has all the details required by CIC, I don't see the need to submit an offer letter. You can include it if you'd like, but its not a requirement. Employment reference letter is gold!! Goodluck.