Imam Ja’far al-Sadiq (a), while explaining the utterance of God Almighty, "That He might try you (to see) which of you is fairest in works." (67:2) said: "It does not mean one of you whose deeds are more numerous but one who is more rightful in his conduct, and this rightness is nothing but fear of God and sincerity of intention (niyyah) and fear." Then he (a) added: "To persevere in an action until it becomes sincere is more difficult than (performing) the action itself, and sincerity of action lies in this that you should not desire anyone to praise you for it except God Almighty, and intention supersedes action. Lo, verily, intention is action itself." Then he recited the Qur'anic verse, "Say, everyone acts in accordance with his character (shakilatihi)", (17:84) adding, "That shakilah means niyyah."
[Al Kulayni, al Kafi, vol. 2, kitab al iman wa al kufr, bab al 'ikhlas, hadith # 4]
Imam al Baqir (a) said: "Perseverance in an action is more difficult than the act itself." He was asked, "What is meant by perseverance in action?" He (a) replied, "A man does some kindness to a relative or expends something for the sake of God, Who is One and has no partner. Thereupon the reward of a good deed performed secretly is written for him. Later, he mentions it to someone and that which was written earlier is wiped out and instead the reward of a good deed performed openly is written for him. Later, when he makes a mention of it again, the vice of riya' (showing off) is written for him (instead of the reward written earlier)." [Al Kulayni al Kafi, kitab al 'iman wa al kufr, bab al riya', hadith # 16]