The Standard of Islam
قَالَ رَسُولُ اللٌّهِ (ص):عَلَمُ الإِسْلاَمِ الصَّلاةُ فَمَنْ فَرَّغَ لَـهَا قَلْبَهُ وَ حَافَظَ عَلَيْهَا بِحَدِّهَا وَ وَقْـتِهَا وَ سُـنَـنِهَا فَهُوَ مُۆْمِنٌ
The Messenger of Allah (peace be upon him and his progeny) said: “The prayer is the standard of Islam. Whosoever loves prayers, and observes their limits, timings and methods, is a true believer.”
Kanzul `Ummal, Volume 7, Tradition 18870
Other links:
The Special Rank of Prayers