Via Translating Jihad:
Shaykh Salih al-Fawzan, a member of the Saudi Council of Senior Scholars, explains in this short fatwa that any Muslim who refuses to pray has become an infidel, and therefore must be killed.
This questioner says, “Your Eminence,” he says, “I have a colleague at work who doesn’t pray. I admonished him, but he didn’t respond. I informed the one in authority over us, and told him, ‘He is afraid of being transferred. Admonish him to pray, and threaten to transfer him.’ My colleague then became angry with me. Here’s my question: Is what I did wrong? What is my duty in this situation?”
He who does not pray is not a Muslim, owing to the saying of Muhammad (PBUH): “(The difference) between worship (of Allah), and infidelity and polytheism, is failing to pray.” He also said (PBUH): “The covenant that is between us and them is prayer, and whoever neglects this has become an infidel.” There are numerous evidences in the Qur’an and Sunnah for the fact that he who fails to pray is an infidel. It is not enough for this man to be transferred, but he must be fired from the job. And if he doesn’t repent to Allah and keep his prayers, then he must be killed. He must be called on to repent, but if he doesn’t repent, then he is killed.