The eminent Muslim scholar, Sheikh Yusuf Al-Qaradawi, states the following: The Muslim is required to perform all the acts of worship: prayer, Zakah, fasting, and Hajj. Scholars of Islam hold different opinions regarding the person who abandons any of these obligations without an excuse.
Some of them declare disbelief on those who abandon any one of them. Some declare disbelief on those who only abandon prayer, because of its status in the religion of Allah and the hadith: “Between a worshiper and disbelief is abandoning prayer.” (Reported by Muslim) Some scholars say that faith and Islam remain with those people as long as they believe in Allah and His Messenger and what he came with, not denying them or doubting them. Then, the final group is those who declare them as sinning transgressors against the laws of Allah.
I think the last view is the most just of the sayings and the closest to the truth. If this is so, then if a person is not constant in carrying out the obligations because of laziness or desire (not denying or belittling them), then his iman (belief) is deficient. But Allah will not put to waste the reward of any action that he does in a good way. For him is goodness for what he has performed, and a sin will be written for what he has neglected.
Allah says: (And everything small and big is inscribed.) (Al-Qamar: 53) and (So whosoever does an atoms weight of good shall see it, And whosoever does and atoms weight of bad shall see it.) (Az-Zalzalah: 78)