It was said to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him): "Allah Almighty mentions prayer frequently in the Qur'an: 'Those who are constant in their prayers' [Surah al-Ma'arij: 23], 'And those who guard their prayers' [Surah al-Ma'arij: 34]." Sayyiduna Abdullah (may Allah be pleased with him) said: "This is about observing the prayer times." The people said: "O Abu Abd al-Rahman, we used to think that it meant not abandoning prayer." Sayyiduna Abdullah (may Allah be pleased with him) said: "Abandoning it is disbelief (kufr)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8906