Allah (swt) says in the Quran:
﴿إِنْ أَحْسَنتُمْ أَحْسَنتُمْ لاًّنفُسِكُمْ وَإِنْ أَسَأْتُمْ فَلَهَا﴾
("If you do good, you do good for your own selves, and if you do evil (you do it) against yourselves.'') Surah 17:7
﴿مَّنْ عَمِلَ صَـلِحاً فَلِنَفْسِهِ وَمَنْ أَسَآءَ فَعَلَيْهَا﴾
(Whosoever does a righteous good deed, it is for (the benefit of) himself; and whosoever does evil, it is against himself.) Surah 45:15
This is a never changing rule with effects that transcend this life into the next.
It makes everything a human being does, with all its results and consequences, totally his own.
Reward is the natural fruit of action. Hence, it is determined by the nature of the action done.
This is to say that a human being is responsible for himself :
It is he who chooses to do good or evil.
He alone who will receive the reward or bear the consequences of what he does.
He has only himself to blame when the result of his action leads him to suffer punishment.
The next statement in Surah Al-Jathiyah (45) ayat 15, Allah (swt) says then to your Lord you will be returned.
This fundamental principle is shown to be central in the way the universe came into existence and in the way it is maintained; meaning you will all return to Allah (swt) on the Day of Ressurrection, when you and your actions will be displayed before Him. Then, He will recompense you for your deeds, good for good and evil for evil.
This is further clarified as the surah responds to those who do evil yet consider that they will be on an equal footing with those who believe in Allah and do good deeds.
وَخَلَقَ اللَّهُ السَّمَاوَاتِ وَالْأَرْضَ بِالْحَقِّ وَلِتُجْزَىٰ كُلُّ نَفْسٍ بِمَا كَسَبَتْ وَهُمْ لَا يُظْلَمُونَ
Allah says in the Qurah that, Allah has created the heavens and the earth with truth, in order that each person may be recompensed what he has earned, and they will not be wronged (Surah 45:22). Allah, the Creator of people’s hearts and the One who has bestowed this Quran from on high, sometimes addresses people in a strong and hard way, at other times with gentle touches, and then again sometimes with a soft and quiet beat. He takes into account their different conditions, circumstances, attitudes and responses. His knowledge is absolute and His wisdom infinite.
"Allah only wants from you your intention and will." If the deed is done for Allah alone, then it is called a sincere deed. This is when it is done for Allah alone without anyone else having any portion of it. If it is done for other than Allah, then it is called showing off, hypocrisy or other than that."
All people are going to be fall into destruction except those of knowledge.
And all of them are going to fall into destruction except those who act (upon their knowledge).
And all of them are going into destruction except those who are sincere.
The first duty upon every servant who wants to obey Allah is to know his intention, and then he has to correct it through acting upon it, after knowing the reality of truthfulness and sincerity.
To act without having an intention is hard work (that brings no benefit), having the intention without sincerity is showing off and sincerity without faith is a waste. Actions are of three types:
1) Sins. Having a good intention while committing a sin does not make it a good deed, rather if you have a bad intention your sin will be greater.
2) Permissible things. There is nothing that is permissible except that a person has one or more intention for doing it and it is possible to make it into an id pod deed.
3) Good deeds. They depend on a person's intention concerning their acceptance and being multiplied. If a person intends to show off then they become sins and minor shirk, which could lead to major shirk.
There are three types of showing off:
1) When showing off is the basic reason for doing a deed, this is shirk and the deed is destroyed.
2) The deed is done for Allah, but then the intention of showing off enters into it. If the end of the deed is not based on the beginning of it, like charity then the beginning of it is accepted and the end is rejected. If the end of the deed is based on the beginning, like prayer then there are two possible situations here:
a. That the person fights against this intention to show off and turns away from it; it will not harm him.
b. That the person submits to his desires; this will destroy the good deed he was performing.
3) That one fears after the completion of the deed that he did it to show off. This is merely whispering (from Satan) and has no effect on the deed or the one who does it.
There are other inconspicuous ways, which may lead one to show off in one's re actions, so you must know them and beware of them.
If a person's intention is to gain some worldly benefit, then his reward or sin or depends on his intention.
There are three types of intentions:
1) The deed done to gain some worldly benefit, such as a person who leads people in prayer to just get money. This person has committed a sin.
The Prophet (saw) said: "Whoever learnt knowledge by which the Face of Allah is sought, but does not do so except for attaining a goal of this world, (he) will not smell the fragrance of Paradise on the Day of Ressurrection." (Abu Dawud)
2) The deed is done for Allah as well as to gain some worldly benefit. This person's belief and sincerity are decreased, such as a person who performs hajj for the sake of trade and to perform hajj. This Persian reward is based on his sincerity.
3) The deed is done for Allah only but the person takes some money or something to help him perform this deed. This person's reward is complete and what he took does not have an effect.
The Prophet (saw) said: "That for which you are most entitled to take a wage is the Book of Allah." (Al-Bukhari)
Know that the people who are sincere in their deeds are on one of three levels:
1) Those who do good deeds to get rewarded or out of fear of being punished. This is the lowest level.
2) Those who do them out of being grateful to Allah and to obey Him. This is the middle level.
3) Those who do them out of love and glorification of Allah. This is the level of the Siddeeqeen (the truthful ones) and it is the highest level.
Rest Allah knows best.
Source: Quran, Sunnah, Tafsir Ibn Kathir, In the shade of the Qur'an, An Explanation of the Last Tenth of the Noble Qur'an [Also with Critical Matters in the Life of a Muslim] Paperback – 2007; www.tafseer.info