If it is still morning, say "I feel unwell. I need to take a day's sick leave".
If it is no longer morning, say "I felt unwell this morning. That's why I didn't come in. I need to take a day's sick leave".
In most companies, you have to notify them as quickly as possible if you're not coming in so I would expect you to say/write "I'm unwell. I can't come in. I need to take a day's sick leave".