Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

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 prayer. If it is complete, then the voluntary (prayers) will also be recorded for him (as an increase). If it is not complete then Allah will say to His angels: 'Look and see whether you find any voluntary prayers for My slave, and take them to make up what is lacking from his obligatory prayers.' Then all his deeds will be reckoned in like manner."