Ibn Abee Dunyaa [rahimahullaah] reported with a chain of transmission going back to Rabee Bin Anas [rahimahullaah], who said: Ikhlaas [Sincerity] to Allaah is the distinguishing sign of the Religion and the distinguishing sign of knowledge is Khashyah [i.e. that fear of Allaah founded upon knowledge of the greatness of Allaah and Him being the owner of complete and perfect sovereignty and Dominion[1].”

Source: Al-Ikhlaas Wan-Niyyah’ page 33′ by Ibn Abee Dunyaa [rahimahullaah]

[1] The short meaning of Khashyah was taken from one of Ustaadh Abu Talhah’s translations [may Allaah grant him and his wife] Jannatul Fir’daws]