Sincerity has been interpreted as being upright, sincere, truthful, pure, distant from show and ostentation in one’s intention and conduct, and being closed to the things that cloud or foul one’s heart. Purity of intention, straightforwardness in thoughts, not pursuing any worldly purposes in relations with Allah, and loyalty in servitude to Him are also included in the meaning of sincerity.
The Muslim believes in the great importance of intention and its importance for the remainder of his deeds, both of this world and the Hereafter. This is because all deeds are based on intention. Depending on the intention, the deed is either valid or void.
This belief in the necessity of having an intention for every deed, and the obligation to make the intention proper, is based on Allah’s Words (which mean): “And they were not commanded except to worship Allah (being) sincere to Him in religion, inclining to truth, and to establish prayer and to give zakah (poor due). And that is the correct religion.” [Quran 98:5]
Allah also said (what means): “Say (O Muhammad): ‘Indeed, I have been commanded to worship Allah (being) sincere to Him in religion.'” [Quran 39:11]
This belief is also based on the words of Allah’s Messenger sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) : “Verily, all actions are but driven by intention and for everyone is what he intended.” [Al-Bukhaari and Muslim]
Allah’s Messenger sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) also said: “Certainly, Allah does not look at your shapes (appearance) or wealth. But He only looks at your hearts and deeds.” [Al-Bukhaari and Muslim]
The one who has a sincere intention is rewarded for a good deed and the one who has an evil intention will bear its burden, just like the one who did a sinful deed. All of that is due to intention alone.
At the Battle of Tabook, Allaah’s Messenger sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) said: “There are people concerning whom you do not travel any distance, nor do you spend anything, nor do you pass any valley but they are with you in that matter.” The people said, “How is that?” He sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) said, “They have been restrained due to some excuse, but they are with us because of the good intention.” [Al-Bukhaari and Abu Daawood]
Sincere intention is what makes the non-combatant equal in reward to the combatant. It is what makes the non-Mujahid receive a reward like that of a Mujahid (one who fights for the sake of Allah).
Allah’s Messenger sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) has also said: “If two Muslims meet each other with their swords, then both the killer and the killed will be in the Hell-fire.” Someone said, “O Allah’s Messenger, that is the case for the killer but why should that be the case for the killed?” He answered, “Because he wanted to kill his companion.” [Al-Bukhaari and Muslim]