It is narrated from the Prophet of Islam (PBUH) who said: A sincere person has four signs: His/her heart is pure, his/her body parts are healthy (without impurity; body parts are not used for bad acts), people are immune from being harmed by such a person and he/she is a good-doer [1]. Imam Ali (AS) said:

The one whose behaviour and saying is the same in hiding and in open, has made his/her worship pure [2]. Imam Ali (AS) also said: The height of sincerity is to avoid sins [3]. In another narration, Imam Ali (AS) said: The pure worship is that the human being puts all his/her trust in Allah (SWT) and no one else; and does not fear anything except his/her sins [4]. Imam Sadeq (AS) also said: The pure deed is the deed which you do not want to worship anybody else except God with it [5].

[1] Thaful Oqool, Page 16.

[2] Nahjul Balaghah, Letter 26.

[3] Beharul Anwaar, Vol. 74, Page 213.

[4] As Above, Page 229.

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

Source: en.shafaqna.com