"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  

"I arrived in Al-Madinah and said: 'O Allah, make it easy for me to find a righteous companion.' Then I sat with Abu Hurairah, may Allah be pleased with him, and said: 'I prayed to Allah to help me find a righteous companion.' So tell me a Hadith that you heard from the Messenger of Allah ﷺ, so that Allah might benefit me from it. He said: 'I heard the Messenger of Allah ﷺ say: "The first thing for which a person will be brought to account will be his Salah. If it is sound then he will have succeeded, be salvaged, but if it is not then he will have lost and be doomed." - (One of the narrators) Hammam said: "I do not know whether this was the words of Qatadah or part of the report." - "If anything is lacking from his obligatory prayers, He will say: 'Look and see whether My slave has any voluntary prayers to make up for what is deficient from his obligatory prayers.' Then all of his deeds will be dealt with in like manner." (Using translation from Nasāʾī 465)  

قَدِمْتُ الْمَدِينَةَ قُلْتُ اللهُمَّ يَسِّرْ لِي جَلِيسًا صَالِحًا فَجَلَسْتُ إِلَى أَبِي هُرَيْرَةَ قُلْتُ إِنِّي دَعَوْتُ اللهَ أَنْ يُيَسِّرَ لِي جَلِيسًا صَالِحًا فَحَدِّثْنِي بِحَدِيثٍ سَمِعْتَهُ مِنْ رَسُولِ اللهِ ﷺ لَعَلَّ اللهَ أَنْ يَنْفَعَنِي بِهِ قَالَ سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «أَوَّلُ مَا يُحَاسَبُ الْعَبْدُ بِصَلَاتِهِ فَإِنْ صَلَحَتْ فَقَدْ أَفْلَحَ وَأَنْجَحَ وَإِنْ فَسَدَتْ فَقَدْ خَابَ وَخَسِرَ» قَالَ هَمَّامٌ لَا أَدْرِي هَذَا مِنْ كَلَامِ قَتَادَةَ أَوْ مِنَ الرِّوَايَةِ وَإِنِ انْتَقَصَ مِنْ فَرِيضَتِهِ شَيْئًا قَالَ انْظُرُوا هَلْ لِعَبْدِي مِنْ تَطَوُّعٍ؟ فَيُكَمَّلُ مَا نَقَصَ مِنَ الْفَرِيضَةِ ثُمَّ يَكُونُ سَائِرُ عَمَلِهِ عَلَى نَحْوٍ مِنْ ذَلِكَ