Allah, the Exalted, says:
"Has not the time come for the hearts of those who believe (in the Oneness of Allah - Islamic Monotheism) to be affected by Allahs Reminder (this Quran), and that which has been revealed of the truth, lest they become as those who received the Scripture [the Taurat (Torah) and the Injeel (Gospel)] before (i.e., Jews and Christians), and the term was prolonged for them and so their hearts were hardened?" (57:16)
"And We sent `Isa (Jesus) - son of Maryam (Mary), and gave him the Injeel (Gospel). And We ordained in the hearts of those who followed him, compassion and mercy. But the monasticism which they invented for themselves, We did not prescribe for them, but (they sought it) only to please Allah therewith, but that they did not observe it with the right observance." (57:27)
"And be not like her who undoes the thread which she has spun, after it has become strong..." (16:92)
"And worship your Rubb until there comes unto you the certainty (i.e., death)." (15:99)
And there are Ahadith, one of them is narrated by `Aishah (May Allah be pleased with her): Messenger of Allah (PBUH) liked that act of worship most in the performance of which a person was regular and constant.