"The first thing for which a person will be brought to account will be his Salah. If it is complete (all well and good), otherwise Allah will say: 'Look and see if My slave did any voluntary prayer.' If he is found to have done voluntary prayers, his obligatory prayers will be completed therewith." (Using translation from Nasāʾī 467)
عَنْ رَسُولِ اللهِ ﷺ قَالَ إِنَّ أَوَّلَ مَا يُحَاسَبُ بِهِ الْعَبْدُ صَلَاتُهُ فَإِنْ كَانَ أَكْمَلَهَا وَإِلَّا قَالَ اللهُ انْظُرُوا أَلِعَبْدِي مِنْ تَطَوُّعٍ فَإِنْ وُجِدَ لَهُ تَطَوُّعٌ قَالَ أَكْمِلُوا بِهِ الْفَرِيضَةَ