Sayyiduna Tamim al-Dari (may Allah be pleased with him) narrates that he (peace and blessings be upon him) said: "On the Day of Resurrection, the first thing a servant will be questioned about is the prayer. If his prayer turns out to be complete, it will be recorded as complete; and if his prayer is not complete, Allah Almighty will say to His angels: See whether this servant of Mine has any voluntary prayers. If he has, then make up the shortfall in the obligatory prayers with those voluntary ones. Then zakah will be reckoned in the same way, and then all deeds will be reckoned according to this same principle."
Hammad bin Salamah narrated it as marfu' (from the Prophet), while others narrated it as mawquf (as the words of the Companion).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4002