"إِنَّ أَوَّلَ مَا يُحَاسَبُ بِهِ الْعَبْدُ يَوْمَ الْقِيَامَةِ مِنْ عَمَلِهِ صَلَاتُهُ. فَإِنْ صَلُحَتْ فَقَدْ أَفْلَحَ وَأَنْجَحَ، وَإِنْ فَسَدَتْ فَقَدْ خَابَ وَخَسِرَ، فَإِنْ انْتَقَصَ مِنْ فَرِيضَتِهِ شَيْءٌ قَالَ الرَّبُّ عَزَّ وَجَلَّ: انْظُرُوا هَلْ لِعَبْدِي مِنْ تَطَوُّعٍ فَيُكَمَّلَ بِهَا مَا انْتَقَصَ مِنْ الْفَرِيضَةِ، ثُمَّ يَكُونُ سَائِرُ عَمَلِهِ عَلَى ذَلِكَ".
On the authority of Abu Hurayrah (may Allah be pleased with him) from the Prophet (peace and blessings of Allah be upon him), who said:
"Allah (mighty and sublime be He) says:
'The first of his actions for which a servant of Allah will be held accountable on the Day of Resurrection will be his prayers.' If they are in order, then he will have prospered and succeeded; and if they are wanting, then he will have failed and lost. If there is something defective in his obligatory prayers, the Lord (glorified and exalted be He) will say: 'See if My servant has any supererogatory prayers with which may be completed that which was defective in his obligatory prayers.' Then the rest of his actions will be judged in like fashion."
[At-Tirmidhi (also by Abu Dawud, an-Nasa'i, Ibn Majah and Ahmad).]