Musnad Abu Ya'la al-Mawsili 132:2

Abdullah ibn Umar ibn Aban narrated to us, Abd al-Rahim narrated to us, from Ash'ath ibn Siwar, from Salamah ibn Kuhayl al-Tan'i, from Amir, from Anas ibn Malik, who said: "Indeed, the first thing the servant will be held accountable for on the Day of Resurrection is the prayer. If it is complete, the rest of his deeds will be complete. And if it is deficient, it will be said: 'Look, does he have any voluntary [prayers]?' If he has voluntary [prayers], He will say: 'Complete with them what was deficient from his prayer.'"