As-salamu `alaykum wa rahmatullah
قُلْ هَلْ نُنَبِّئُكُم بِالْأَخْسَرِينَ أَعْمَالًاالَّذِينَ ضَلَّ سَعْيُهُمْ فِي الْحَيَاةِ الدُّنْيَا وَهُمْ يَحْسَبُونَ أَنَّهُمْ يُحْسِنُونَ صُنْعًا
“Say: Shall We tell you the greatest losers in respect of (their) deeds? Those whose efforts have been wasted in this life while they thought that they were acquiring good by their deeds!” [al-Kahf: 103-104]
This is a thought-provoking verse that should get us to really reflect and contemplate. To get the best out of the Qur’an, one should read it as if it were a personal letter from their Lord. Ponder over every verse and see the various ways that it can apply to you. Does it offer guidance that you can follow? Does it contain a warning that you can pay heed to? Does it offer a new way of thinking that can expand your horizon? Any way we can apply a verse to our own selves, let’s apply it. That is when the Qur’an will create a dramatic change within us.
Subhan’Allah, one of the worst things that can happen to a person is for them to come on the Day of Judgement with deeds upon deeds, only to then realise that those deeds have mounted to nothing. Or that the reward has been diminished greatly. This can happen due to us failing in sincerity or intention…
The Prophet (sallallahu `alayhi wa sallam) said, “People will be resurrected upon their intentions.” 
Sincerity is an act of the heart and one of its most important… Acts of the heart are generally more crucial than outward acts of the limbs (prayer, fasting, Hajj etc) because the heart needs to submit to Allah before the body can submit.
Ibn al-Qayyim said, “Whoever contemplates over the Shari’ah, its source and ways, will come to know how the acts of the limbs are tied to the acts of the heart and that they do not benefit without them. He will also come to know that the acts of the heart are more obligatory upon the slave than the acts of the limbs. Is the believer distinguished from the hypocrite except by virtue of what is in the heart of each of them when it comes to deeds? Worship of the heart is greater than the worship of the limbs and it is more abundant and lasting, hence it’s mandatoryat all times.” 
Interestingly, the salaf used to teach intentions and sincerity just like they taught other acts of worship.
Sufyan al-Thawri said, “They used to learn about intentions just like they used to learn about deeds.” 
We also ought to teach how to have a good intention, how to rid oneself of riya’, how to be sincere and how to recognise the corrupting factors which lead to bankruptcy on the Day of Accounting.
Tustari: “Intention is sincerity. Just as the outer reality (of a person) is made steadfast by good deeds, the inner reality is made steadfast by intentions. Whoever does not know his intention, does not know his Deen (religion). And whoever neglects his intention will fall into a state of confusion.”
Yahya ibn Abi Kathir: “Learn about intentions because they travel further than deeds.” 
Yusuf ibn Asbat said to Bashar, “Learn how to differentiate between a good deed and a bad deed. Indeed, it took me 22 years to learn the difference.” 
Sulayman al-Dariani: “Glad tidings to the one who proves sincere in one step which he takes desiring only Allah, the Most High.”
Abu Talib al-Makki: “A righteous intention is one’s first good deed; it is the first gift of Allah (to His slave) and it is the first point of reward. A slave will only get the reward of a deed according to what Allah has gifted him with of intentions, and perhaps a single deed will gather numerous intentions according to the slave’s capacity and his knowledge, so for every intention he receives a reward and each is multiplied tenfold because they (intentions) are deeds which have been gathered within a deed.” 
Ibn al-Qayyim: “Ikhlas (sincerity) is that which no Angel knows of such that he can write it down, and no enemy is aware of such that he can corrupt it, and the individual is not amazed at it such that he renders it invalid.” 
Al-Susi: “Sincerity means to lose sight of your sincerity, for indeed the one who bears witness to his sincerity, then his sincerity is in need of sincerity!”
May Allah grant us sincerity and protect us from everything that will corrupt us, ameen.
1] Reported by Ahmad, declared Sahih by al-Albani in Sahih al-Jami’
2] Bada’i al-Fawa’id, by Ibn al-Qayyim
3] Qut al-Qulub, by Abu Talib al-Makki
4] Hilyat al-Awliya’, by Abu Nu’aym
5] See above source
6] Qut al-Qulub, by Abu Talib al-Makki7] al-Fawa’id by Ibn al-Qayyim
7] al-Fawa’id by Ibn al-Qayyim