Whoever denies that the five daily prayers are obligatory is a disbeliever by consensus, because he rejects something necessarily known of the religion. As for one who affirms the obligation but abandons prayer out of laziness, there is genuine ikhtilaf: the majority — the Hanafis, Malikis and Shafi'is — hold that he is a grave sinner who remains a Muslim (the Malikis and Shafi'is adding that he is executed as a hadd punishment yet is washed, prayed over and buried as a Muslim), while the well-known position of Imam Ahmad — followed by Ibn Taymiyyah and by contemporary scholars such as Ibn Baz and Ibn 'Uthaymin — is that this is major kufr, on the strength of the hadith of Jabir, "Between a man and shirk and kufr is the abandonment of prayer" (Muslim 82). Both sides agree that leaving prayer is among the very greatest sins and a mark of the people of the Fire, who when asked what put them into Saqar said, "We were not of those who prayed" (74:42-43), and that those who came after the prophets and "neglected the prayer and followed desires" are warned of ruin (19:59). Practically the ruling is the same on either view: return to the prayer at once and repent, rather than argue over the label.
Q&A · Belief
Is a Muslim who does not pray a disbeliever?
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.