Sincerity is achieved when Muslim brothers or sisters have the intention to please Allah SWT in all their endeavors whether in speech or action. The Muslim seeks generous rewards from Allah SWT without considering any other motives. He or she does not try to gain worldly favors in terms of self promotion or personal interests. In this way, the Muslim will always serve the Islamic faith and thought, rather than being subservient to short term benefits.
Hence, sincerity is the first key that opens the heart to receive the Mercy of Allah, subhanahu wa ta'ala. In fact the acceptance of one's deeds depends on one's purity of intention, which in turn depends on the level of sincerity in one's heart. With sincerity, a person can attain the highest ranks in the sight of Allah SWT, make the most of his/her deeds, and even be rewarded for righteous deeds that he or she was not able to perform. On the other hand, without sincerity (which results from not taking care of the heart) a person may end up in Hellfire regardless of how righteous one's deeds appear to be.

What is the evidence for this statement?
1) Al-Hakim reported in Al-Mustadrak that Anas ibn Malik, radhiy Allahu anhu, narrated that Prophet Muhammad (pbuh) said: "Whoever leaves this world with sincerity to Allah SWT Alone, Who has no Partner, and performs the prayers and pays the poor dues (zakat) will die in the pleasure of Allah.
2) Authentic, under the conditions of Bukhari and Muslim, is also Imam Muslim-report from Abu Huraira that the Messenger of Allah (pbuh) said: "Three categories of people will be the first to be thrown into Hellfire: a scholar who taught people merely to be praised as a scholar; a generous person who gave charity so that people will call him generous; and a person who participates in Jihad to be recognized as brave."
In order to avoid losing the rewards of our deeds, we should pay attention to our hearts in three stages:
First, before starting to do anything, ask yourself: am I doing this to please Allah (SWT) and to seek his forgiveness and reward, or am I doing it to impress people and seek their praise? If you have any doubt about the sincerity of that task, then you should renew your intention and free it from anything that would interfere with sincerity. On the other hand, if it is sincere, you will feel ease and comfort deep in your heart, and therefore you should go ahead with that task with full confidence.

Second, while still engaged in a given task, a person may encounter interference from his vain desires or Satan. One may like to be seen while engaged in that task. There is nothing wrong with people seeing you while doing a task for the sake of Allah (SWT). But if love of publicity and praise dominates one's actions, then there is cause for concern because this may lead to the rejection of that deed by Allah (SWT). A good indication of sincerity is the similarity of a person's behavior in the presence of people and in their absence.

Third, after completing an assignment or a job, do not continue to boast about your role in it. This feeling becomes particularly obvious if somebody wrongs you or someone is chosen for a position you were seeking, and you resort to hurting that person by defaming him. This course of action destroys your good deeds and will turn out to be detrimental to you.
In this verse Allah (SWT) equates showing off with boasting about what one did for others.
Here are some criteria that indicate to us whether we are on the right track to sincerity and the pleasure of Allah (SWT).

Signs of Sincerity

To have good intentions before starting a task.
Habits are changed by acts of worship
To perfect one's duty in private and in public.
To dislike being praised by others, with du'aa: "O Allah, forgive me for what they do not know about me, and make me better than what they think of me."
To welcome advice from friends.
To be visible only at times of ease and unavailable when help is needed.
To not seek leadership and not nominate yourself for positions of authority.
To be constantly mindful of one's shortcomings, and to advise or wish others well.
To feel that your contribution is minimal compared to others.
To love performing the Fajr, Ishaa', and night prayers.
To prefer giving charity in secret.

Signs of Lack of Sincerity

Starting a task without having good intentions.
Acts of worship change into habits.
Perfecting one's duty only in public.
Feeling happy in being praised by others, and forgetting about the supplication.
Feeling to advice from others.
Being available at times of difficulty less than at times of ease.
Seeking and campaigning for votes in order to be selected to a position of authority.
Exaggerating one's achievements and diminishing the achievement of others.
Feeling that you are more hardworking than others.
Find these prayers difficult to perform.
Preferring to give charity openly in order to be seen by others.

These are some of the signs of sincerity or lack of it. We therefore have to exert ourselves to achieve sincerity because it is a great loss to work hard and end up losing everything. As for those with sincere hearts, they will attain the highest levels of Paradise. Allah describes them: "Say: 'Verily, I am commanded to serve God with sincere devotion; and I am commanded to be the first of those who bow to God in Islam'" (39:11-12). Being the first of Muslims means realizing happiness in this world and the highest levels of Paradise in the Hereafter.
Therefore make a solemn promise to Allah to make your soul display sincerity, such that it becomes a natural part of your existence. In this way, we will possess the key to the heart, which subsequently makes our acts of worship a very pleasant experience. Only those who are endowed with this quality can feel its effect on the person. Finally, I ask Allah the Almighty to make us among the sincere, who are mindful of Allah without others knowing about it. The people with this characteristic perform their duties without others knowing who they are. On the other hand, when they are absent, people don't know much about them since they do not care about publicity. They are the keys to guidance whom Allah uses to guide those who have gone astray.
We seek refuge with Allah to save us from the vain desires of the soul and the temptations of Satan.

Dr. Salah Soltan

Director of the Islamic Education and Fatwa Center, ISGC - President of the American Center for Islamic Studies