“The meaning of sidq (truthfulness) is that the information is in conformity with reality, if this is the case then it is the truth and if it opposes the reality then it is kadhib (a lie/falsehood). Truthfulness is in statements and likewise in actions, i.e., that which is done outwardly is in conformity with what is inside (of belief etc), so that when a person performs an action it is in conformity with what is in his heart.

The one who does actions to show off then he is not truthful, because he shows the people that he is from the (sincere) worshippers, and this is not the case.

The mushrik (one who associates partners with Allah) is not truthful, because he makes apparent that he is a muwahhid (one who worships Allah alone upon Tawhid) and this is not the case.

The munaafiq (hypocrite) is not truthful because he makes apparent Eemaan (in Allah, His Book, His Messenger, His Angels, The Last Day, Pre-Decree the good of it and the bad of it), whilst he is not a believer.

The mubtadi’ is not truthful because he makes apparent that he is following the Messenger (sallallaahu alaihi wa salam) however he is not a follower (of the sunnah).

That which is important is that truthfulness is that which is in conformity with reality, and it is one of the characteristics of the believers and it’s opposite, lies and falsehood are from the characteristics of the hypocrites, refuge is sought with Allah.” (1)

Shaykh al-Uthaymeen in explanation of the introduction to the chapter on Truthfulness. Sharh Riyadh as-Saaliheen v.1 pg. 289-290