The difference between a believer and an unbeliever is whether he/she says his/her prayers or not
Sahih Muslim Book 1 #147
Full Hadith
حَدَّثَنَا أَبُو غَسَّانَ الْمِسْمَعِيُّ، حَدَّثَنَا الضَّحَّاكُ بْنُ مَخْلَدٍ، عَنِ ابْنِ جُرَيْجٍ، قَالَ أَخْبَرَنِي أَبُو الزُّبَيْرِ، أَنَّهُ سَمِعَ جَابِرَ بْنَ عَبْدِ اللَّهِ، يَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ “ بَيْنَ الرَّجُلِ وَبَيْنَ الشِّرْكِ وَالْكُفْرِ تَرْكُ الصَّلاَةِ ”
It is narrated on the authority of Abu Zubair that he heard Jabir bin ‘Abdullah saying, “I heard the Messenger of Allah (may peace and blessings be upon him) say:
“Between man and polytheism and unbelief is the abandonment of salat.”
Lesson
The most important lesson to take from this hadith is that prayer should be observed with care. Missing prayers is such a grave sin, to the extent that the Prophet Sallallahu Alayhi Wasallam has clearly stated that prayer is what distinguishes a believer from an unbeliever. Therefore, we should pray properly and on the right time.