Mishkat al-Masabih 4 Prayer كتاب الصلاة 10

وَعَنْ بُرَيْدَةَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «الْعَهْدُ الَّذِي بَيْنَنَا وَبَيْنَهُمْ الصَّلَاةُ فَمَنْ تَرَكَهَا فَقَدْ كَفَرَ» . رَوَاهُ أَحْمَدُ وَالتِّرْمِذِيّ وَالنَّسَائِيّ وَابْن مَاجَه

Buraida reported God’s Messenger as saying, “The covenant between us and them is prayer, so if anyone abandons it he has become an infidel.” Ahmad, Tirmidhi, Nasa’i and Ibn Majah transmitted it.