Sunan an-Nasa'i, Book 5, Hadith 20 (In-Book) ⓘ

It was narrated from Abu Hurairah that the Messenger of Allah said: "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."

أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا النَّضْرُ بْنُ شُمَيْلٍ، قَالَ أَنْبَأَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنِ الأَزْرَقِ بْنِ قَيْسٍ، عَنْ يَحْيَى بْنِ يَعْمَرَ، عَنْ أَبِي هُرَيْرَةَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ أَوَّلُ مَا يُحَاسَبُ بِهِ الْعَبْدُ صَلاَتُهُ فَإِنْ كَانَ أَكْمَلَهَا وَإِلاَّ قَالَ اللَّهُ عَزَّ وَجَلَّ انْظُرُوا لِعَبْدِي مِنْ تَطَوُّعٍ فَإِنْ وُجِدَ لَهُ تَطَوُّعٌ قَالَ أَكْمِلُوا بِهِ الْفَرِيضَةَ ‏" ‏ ‏.‏