Musannaf Ibn Abi Shayba 34:172

Yazid ibn Harun narrated to us, from Dawud, from Zurarah ibn Awfa, from Tamim al-Dari, who said: The first thing the servant will be held accountable for on the Day of Resurrection is the obligatory prayer. If he completed it, [it is accepted]; otherwise, it will be said: "Look to see if he has any voluntary [prayers]." Then the obligatory [prayer] will be completed from his voluntary [prayers]. If the obligatory [prayer] is not completed and he has no voluntary [prayers], he will be seized by his two sides and cast into the Fire.