Sheikh Al ‘Uthaymīn (May Allāh have mercy on him) said:

Prayer is the greatest pillar of Islam after the two testimonies of faith. If somebody abandons it out of negligence on laziness, that makes him a disbeliever. If he denies that it is obligatory, that makes him a disbeliever - even if he prays. [Al-Sharḥ Al-Mumti’ ‘Alā Zād Al-Mustaqni’ vol.3 p.4]