Prophet Muhammad (PBUH) was narrated to have said: “There is an essence to everything: a soul, a significance, a quintessence intrinsic to it. A person cannot reach that essence of sincerity (Ikhlas), the true significance of sincerity [It is not easy to gain; it is a virtue of a very high level.], until they do not accept admiration from others regarding any deed that they perform for the sake of God.*
For instance, imagine a person that does not perform good deeds for the sake of attracting others’ attention; they perform prayers for the sake of God: they read through the Quran, do some good deeds, help others with some charitable donations or assistance, and fight on the path of God. Yet, they enjoy it when others praise them by saying, “This is a great person.” --They did not intend to do these actions for the people, he or she has done it for God; but, they are pleased with the admiration from others. --It is said that this is not the elevated level of sincerity (Ikhlas).
The elevated level of sincerity is gained when the faithful do not enjoy the people’s admiration. They don’t care about others’ thoughts! They don’t care if others learn of their good deeds or not! --If they admire them or not! They regard what God requires of them, and they do it appropriately.
Imam Khamenei, February 23, 1993
* Bihar al-anwar, vol.69: 304