Nafi’ reported: Umar ibn al-Khattab, may Allah be pleased with him, wrote to his deputies, saying, “Verily, the most important of your affairs to me is the prayer. Whoever maintains it and watches over it will maintain his religion. Whoever wastes it will be even more wasteful of other things.”
Source: al-Muwaṭṭa’ 6
عَنْ نَافِعٍ أَنَّ عُمَرَ بْنَ الْخَطَّابِ رضي الله عنه كَتَبَ إِلَى عُمَّالِهِ إِنَّ أَهَمَّ أَمْرِكُمْ عِنْدِي الصَّلاَةُ مَنْ حَفِظَهَا وَحَافَظَ عَلَيْهَا حَفِظَ دِينَهُ وَمَنْ ضَيَّعَهَا فَهُوَ لِمَا سِوَاهَا أَضْيَعُ
6 موطأ مالك باب وقوت الصلاة