Prayer
أَوَّلُ مَا يُحَاسَبُ بِهِ الْعَبْدُ يَوْمَ الْقِيَامَةِ الصَّلَاةُ، فَإِنْ صَلَحَتْ فَقَدْ أَفْلَحَ وَأَنْجَحَ، وَإِنْ فَسَدَتْ فَقَدْ خَابَ وَخَسِرَ
Translation
The first matter that the servant will be held accountable for on the Day of Judgment is the prayer. If it is sound, then the rest of his deeds will be sound. And if it is corrupt, then the rest of his deeds will be corrupt.
Narrator
Abu Hurairah
Source
Sunan al-Tirmidhi
Tirmidhi 413