In a small saucepan, bring the apple cider, sugar and cinnamon to a boil. Allow the mixture to boil until the sugar is completely dissolved and then turn the heat to low. Simmer until the mixture becomes thick and syrupy, approximately 5-10 minutes. Allow the syrup to cool and transfer to an airtight container in the refrigerator until ready to use.
The syrup will keep in the refrigerator up to 1 month.