Sahih Muslim 82
Sahih Muslim · Book 1: The Book of Faith · Clarifying the usage of the word Kafir for one who abandons Salat
It is narrated on the authority of Abu Zubair that he heard Jabir b. 'Abdullah saying. I heard the Messenger of Allah (may peace and blessings be upon him) observing this:
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 " .
Between man and polytheism and unbelief is the abandonment of salat.