Musnad Ahmad 1017:3

Ali ibn al-Hasan—meaning Ibn Shaqiq—narrated to us, al-Husayn ibn Waqid narrated to us, Abdullah ibn Buraydah narrated to us, from his father, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "The covenant that is between us and them is the prayer; so whoever abandons it has committed disbelief."