Musannaf Ibn Abi Shayba 23:88

Yahya ibn Wadih narrated to us, from Husayn ibn Waqid, who said: I heard Ibn Buraydah saying: I heard my father saying: 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."