Below is a quotation from the generic list of documents to be submitted after receiving an ITA:
Document: Employment Records
You must provide proof of each of your current and previous work experiences.
Your evidence should include a reference letter from your employer and previous pay stubs, where available.
The reference letter is an official document printed on company letterhead and must include your name, the company's contact information (address, telephone number and e-mail address), the signature of your immediate supervisor or personnel officer at the company and a business card of the person signing the letter. The letter should indicate all positions held while employed at the company and must include the following details: job title, duties/responsibilities, job status (if current job), the dates you worked for the company, the number of work hours per week and your annual salary plus benefits.
You must scan all documents for this work experience and save them as one file. (Each work experience requires a separate file.)
For more information on the documents required, follow this thread: http://www.canadavisa.com/canada-immigration-discussion-board/documents-required-after-ita-t280257.0.html