In Al Quran:
Verily, those who say: `Our Rubb is Allah (Alone),' and then they stand firm, on them the angels will descend (at the time of their death) (saying): 'Fear not, nor grieve! But receive the glad tidings of Jannah which you have been promised! We have been your friends in the life of this world and are (so) in the Hereafter. Therein you shall have (all) that your inner-selves desire, and therein you shall have (all) for which you ask. An entertainment from (Allah), the Oft-Forgiving, Most Merciful.''' (41:30-32)
Sufyan bin `Abdullah (May Allah be pleased with him) reported: I said, "O Messenger of Allah, tell me something of Islam which I will not ask anyone else about it.'' He (PBUH) said, "Say, `I believe in Allah' and then be steadfast".
Steadfastness means adhering firmly to the injunctions of Islam, to act upon what is ordained and avoid what is prohibited, to perform dutifully orders in respect of obligations and what is regarded desirable and avoid what is unlawful. Faith is not a mere expression of Shahadah. The real Faith is one which is coupled with its actual practice because practice of religion is in fact the fruit of Faith. A tree which does not bear any fruit, has no value, similarly; faith becomes meaningless without practice. Steadfastness is the mark of perfect Faith.