Sunan An-Nasai, Chapter: The Book of Salah

that the Prophet (saw) said: "The first thing for which a person will be brought to account on the Day of Resurrection will be his Salah. If it is found to be complete then it will be recorded as complete, and if anything is lacking He will say: 'Look and see if you can find any voluntary prayers with which to complete what he neglected of his obligatory prayers.' Then the rest of his deeds will be reckoned in like manner." (Sahih)