If you are referring to an employee or contract laborer, then you need to contact them to get their ssn for reporting purposes if you are required to issue them a tax reporting document. You should have had them file Form W-9 before being paid so you would have it.

If you don't get their ssn or itin, you cannot issue them a tax reporting document and will be at risk of losing the deduction for the income you paid them, not to mention a fine from IRS for not issuing tax reporting documents when required.