Use a friendly but professional tone in your communication. It’s likely that your employee has expressed an interest in speaking with you, so don’t be overly formal. You should briefly describe the role you’re hiring for and why you believe they’d be interested. Point out how your candidate’s talents and expertise are relevant to your job demand, aside from having a shared link.
As with all candidate sourcing emails, you should indicate what the next steps are (e.g. set up a call) and provide the information your candidate might need e.g. contact details, job description, company website.
Always be professional, but make sure that the email is written in a tone that reflects your company’s culture.