Sincerity (Ikhlas) is the base of worship (Ibadah) and its truth and reality. Ikhlas is the spirit of devotion and servitude. The Prophet of Islam (PBUH) said:

Allah (SWT) will not look at your wealth and faces; rather God will look at your hearts and deeds [1]. It is narrated from Imam Sadeq (AS) who said: Intention (Niyyah) is superior to deed. Beware that Niyyah is the truth and reality of the deed (act) [2]. Therefore, sincerity (Ikhlas) cleanses intention (Niyyah) from polytheism (Shirk) and hypocrisy (Ria).

[1] Mizanul Hekmah, Vol. 3, Page 2601.

[2] Osul Kafi, Vol. 2, Page 16.

Source: en.shafaqna.com