The first thing checked, and what covers the gap
If it is short, they are told to look for something else.
انْظُرُوا، هَلْ تَجِدُونَ لِعَبْدِي مِنْ تَطَوُّعٍ؟ فَأَكْمِلُوا بِهَا مَا ضَيَّعَ مِنْ فَرِيضَتِهِ
Unẓurū, hal tajidūna li-ʿabdī min taṭawwuʿ, fa-akmilū bihā mā ḍayyaʿa min farīḍatih
Look, do you find any voluntary prayers for My servant? Then complete with them whatever he let slip of his obligation.
Where this comes from
The first thing a servant is called to account for is the prayer. If it is complete it is written complete. If something is missing, the order is given to look for the voluntary ones and use them to fill it, and then everything else is taken the same way.
The full narration
It was narrated from Tamim Dari that the Prophet (ﷺ) 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.”
Source
Sunan Ibn Mājah · 1426
- Al-Albani: Sahih
- Muhammad Fouad Abd al-Baqi: Sahih
- Zubair Ali Zai: Isnaad Sahih