What is the reality of ostentation (riyaaʾ)? Is the mere feeling of happiness when people learn of a deed done purely for the sake of Allah considered ostentation? How does a Muslim know if he is being ostentatious? And is ostentation a hidden shirk that nullifies all deeds, or only the deed that was mixed with ostentation?

Showing off (riya') is performing deeds to be seen by people, and seeking reputation (sum'ah) is doing so to be heard by people. This is the opposite of sincerity (ikhlas), which is intended for the sake of Allah. Feeling joy when people become aware of one's acts of obedience is not always blameworthy. If the intention was to conceal the act of obedience for Allah's sake, and the doer felt joy at Allah's grace and kindness in revealing his obedience and concealing his disobedience, then this is praiseworthy. However, if his joy at people becoming aware of his deeds is to gain their praise and admiration, then this is blameworthy. The Prophet, peace and blessings be upon him, was asked about a man who performs a good deed and people praise him for it. He replied: "That is the immediate good tidings for the believer." Showing off is a hidden polytheism (shirk khafi) that nullifies the deed in which it occurred.