It is related that Prophet Muhammad (peace be upon him) said:

“The greatest fear I have for my community is a lesser form of polytheism… showing off.” (Ahmad)

There is nothing wrong with a believer feeling happy on account of people’s praise. It is only showing off when a person intends to do a good deed for other than God’s sake, so that he or she would not do that deed if no one else was there to see it.

Showing off takes many forms:

Showing off in One’s Faith

This is the most serious kind and it is not a “lesser” form of polytheism. Another word for it is hypocrisy. It is to profess belief to the people while concealing disbelief in one’s heart.

Showing off with One’s Appearance

This is to make oneself appear as if one is engaged in a lot of devotions, like to deliberately cultivate a prostration mark on the forehead or to make one’s lips dry so to make it seem like the lips of a fasting person.

It also includes bowing the head in false humility while walking or keeping disheveled hair to appear ascetic.

Showing off Through what One Says

Prophet Muhammad (peace be upon him) said:

“Whoever calls attention to himself, Allah will call attention to how he really is.” (Al-Bukhari and Muslim)

This includes quoting wise sayings, exhorting people to righteousness, and quoting hadith to bolster one’s “pious” reputation. It also includes moving one’s lips to give the appearance of being engaged in God’s remembrance.

Showing off Through One’s Deeds

This includes spending an extra long time standing, bowing or prostrating in prayer when others are watching.

Showing off Through Association

This is like making sure to be seen with prominent scholars and pious people in order to be associated with them in the people’s eyes.

In all cases, the crux of the matter is not the action itself, but what motivates the action in the first place. The motivation for showing off stems from either a desire for praise, an aversion for people’s low opinion, or a covetousness for what other people possess.

If something of this nature comes into a person’s heart while the person is already engaged in an act of piety, then it does not nullify the blessings of that act.

Some people abstain from doing good deeds fearing that they will fall into the sin of showing off. This is a mistake that leads people to lose out on a lot of virtue and many blessings. As long as your original intention is for God, then you should go forward with the good deed you intend. Do not let fear of showing off keep you from doing something good. Al-Fudayl ibn `Iyad said:

“Engaging in acts of worship for the people’s sake is polytheism. Abandoning acts of worship for the people’s sake is showing off. Sincerity is for Allah to spare you from both concerns.”

A student at an early stage rushes ahead and starts issuing Islamic legal verdicts, heads a study circle, and walks about with a regal demeanor. He has a hard time admitting when he does not know something. He speaks as if he is a leading authority, making statements like:

“In my considered opinion…”, “What has become evident to me…”, “I have come to the overwhelming conclusion that…”, “What a person’s heart feels secure with is…”

At the same time, he is harsh and long-winded when he comes across someone else’s mistake, though he cannot tolerate their pointing out any error of his.

Another person gives preference to carrying out public duties at the expense of his own individual obligations. He might even spend excessive time engaged in things that the community might only rarely need, simply because they make him look important.

A third person is overjoyed with any chance he has to argue and debate with people. In the heat of an argument, he is quick to challenge people to invoke Allah’s curse upon whichever one of them are in the wrong. Often, his point of contention is no more than hair-splitting and his only purpose is to publicly expose his opponent as wrong or misguided. It infuriates him when he knows his opponent has made a good point. On the other hand, if his opponent concedes a point to him, he says:

“Now you have come to my point of view and my way of thinking” as if he has had a monopoly on the truth all along and decides who can partake of it with him. In almost all cases, what the Prophet (peace be upon him) said about debates holds true:

“Base motives are obeyed, passions are followed, and each holds fast to his own opinion.” (Abu Dawud and At-Tirmidhi)

A wise man was once asked:

“Why is it that the words of the Pious Predecessors are more beneficial than what we say?”

The wise man replied:

“The words of the Pious Predecessors are better than our words, because they spoke to promote Islam, please Allah, and guide people to salvation, whereas we speak to promote ourselves, please the people, and achieve worldly success.” Some people like to dig up strange opinions and resurrect old arguments to make it seem like they are resuscitating a Sunnah that has been forgotten or neglected.

Our scholars had a different attitude about such things. They warned against strange and unusual opinions that show themselves in their very strangeness to be suspect. It is possible, nevertheless, to go to the opposite extreme. When a well-known custom or tradition is called into question, some people come forward as self-proclaimed defenders of tradition, hoping to earn a prominent position in society by doing so, even if they know that tradition is misguided, does not serve the public welfare, or is contrary to the teachings of Islam.

Another mode of conduct in this vein is to seek after a lot of followers, pit people against one another, erect obstacles to reconciliation, and stake loyalty on adherence to a bunch of secondary controversial opinions. Al-Dhahabi said:

“You can be an oppressor and believe that you are the one who is oppressed. You can be consuming unlawful wealth and fancy yourself to be abstentious. You can be a sinner and think you are righteous and just. You can be seeking religious knowledge for worldly benefit but see yourself as doing so for Allah’s sake.” (Siyar A`lam al-Nubala’)

Abu Dawud, speaking about his Sunan, told Imam Ahmad:

“This is something I have done for Allah’s sake.”

Ahmad said to him:

“As for saying it is for Allah’s sake, that is a serious claim. Rather say:

“This is something my heart has been made to incline towards, so I did it”.” It is also related as something Imam Ahmad admonished himself with.

Indeed, identifying your inner motives is one of the subtests but most crucial ways of being honest.

Source: http://en.islamtoday.net.