Ikhlas requires that one pursue nothing worldly while worshipping and obeying God, that one fulfills the duty of servanthood only because God orders it, and that one remains silent concerning any personal experiences of God's special treatment and special gifts and seek only His approval and pleasure.
Sincerity is one of the most significant qualities of those most faithful or loyal to God; loyalty is regarded as a source, and sincerity as a sweet water originating from it. The most eloquent of humanity, upon him be peace and blessings, declared that one who drinks uninterruptedly from this water for forty days will find channels of wisdom opened from his or her heart to his or her tongue, and that such a person will always speak wisdom.
Loyalty or faithfulness is the primary attribute of Prophethood, and sincerity is its most lustrous dimension. Sincerity is innate in the Prophets; all other people try to obtain it during their lifetime. Among them, for example, the Qur'an describes the Prophet Moses as one made sincere (19:51).
Faithfulness and sincerity were as intrinsic and essential to the Prophets as air and water are to the lives of those who communicate the Prophets' message to others in every age. In addition, they were the Prophets' most important sources of power. The Prophets were convinced that they could not take one step forward without sincerity, and the representatives of the cause of Prophethood must believe that they will be able to achieve nothing without it. Faithfulness and sincerity are two wings or two deep oceans extending from Divine Favor and Grace to an individual's heart. One who can sail in these oceans or fly with these wings will reach the destination, for they are under God's protection. God values that which is done to please Him, regardless of its apparent size or importance, not the quantity of deeds. Therefore, He values a small deed done with sincerity over many deeds done without sincerity.
Sincerity is an attitude of the heart, and God views an individual according to his or her heart's inclination. The Prophet, upon him be peace and blessings, declares: Assuredly, God does not consider your bodies, nor your appearances. Rather, He considers your hearts.  Sincerity is a mysterious Divine credit granted to those who are purehearted in order to increase what is little and to deepen what is shallow, and to give finite (limited) worship infinite reward. One can use it to purchase the most valuable things in the markets of this world and the next, for it is esteemed, welcomed, and respected where others suffer great misery. This mysterious power of sincerity caused God's Messenger, upon him be peace and blessings, to declare: Be sincere in your religion; little work (with sincerity) is enough for you,  and: Be sincere in your deeds, for God only accepts what is done with sincerity. 
If we consider a deed to be a body, sincerity is its soul. If a deed represents one wing of pair of wings, sincerity is the other. A body without soul is of no worth, and nothing can fly with only one wing. How fine are Mawlana Jalal al-Din al-Rumi's words:
You should be sincere in all your deeds,
So that the Majestic Lord may accept them.
Sincerity is the wing of the bird of the acts of obedience.
Without a wing, how can you fly to the abode of prosperity?
The following words of Bayazid al-Bistami are also very apt:
I worshipped my Lord for thirty years with all my strength. Then I heard a voice saying: O Bayazid! The treasures of God Almighty are full of acts of worship. If you intend to reach Him, see yourself as small at the door of God and be sincere in your deeds.
For some, sincerity involves hiding from others when performing supererogatory deeds and avoiding all show and ostentation. For others, it means that whether one is or is not seen while performing religious deeds is not important. Still for others, it means being so involved in worship or religious deeds in consideration of God's pleasure that one does not even remember whether one should be sincere or not.
Self-supervision is an essential dimension of sincerity, and a truly sincere person does not consider any possible spiritual pleasure that may be derived, or speculate upon whether it will ensure entrance to Paradise. Sincerity is a mystery between God and a servant, and God puts it in the hearts of those He loves. One whose heart is awakened to sincerity does not worry about being praised or accused, exalted or debased, aware or unaware of deeds, or being rewarded. Such a person does not change, and behaves in the same way in public and in private.
 'Abd al-Ra'uf Munawi, Fayd al-Qadir, 6 vols. (Beirut 1093 ah / 1682 ce) 1:216.
 Ibid., 1:217.