Musannaf Ibn Abi Shayba 23:130

Isma'il ibn 'Ayyash narrated to us, from 'Ubayd Allah ibn 'Ubayd al-Kala'i, who said: He took the hand of Makhul and said: "O Abu Wahb, let the matter of faith be great in your soul; whoever abandons a prescribed prayer intentionally, the protection of Allah is absolved from him, and whoever the protection of Allah is absolved from, he has disbelieved."