The word(s) "truthful" appears 45 time(s) in 44 verse(s) in Quran in Muhammad Pickthal translation.

(31) Say (unto them, O Muhammad): Then bring a scripture from the presence of Allah that giveth clearer guidance than these two (that) I may follow it, if ye are truthful.

(32) And they say: When cometh this victory (of yours) if ye are truthful?

(33) And they say: When is this promise (to be fulfilled) if ye are truthful?

(34) And they say: When will this promise be fulfilled, if ye are truthful?

(35) Then produce your writ, if ye are truthful.

(36) And a believing man of Pharaoh's family, who hid his faith, said: Would ye kill a man because he saith: My Lord is Allah, and hath brought you clear proofs from your Lord? If he is lying, then his lie is upon him; and if he is truthful, then some of that wherewith he threateneth you will strike you. Lo! Allah guideth not one who is a prodigal, a liar.

(37) And when Our clear revelations are recited unto them their only argument is that they say: Bring (back) our fathers. then, if ye are truthful.

(38) Say (unto them, O Muhammad): Have ye thought on all that ye invoke beside Allah? Show me what they have created of the earth. Or have they any portion in the heavens? Bring me a scripture before this (Scripture), or some vestige of knowledge (in support of what ye say), if ye are truthful.

(39) They said: Hast come to turn us away from our gods? Then bring upon us that wherewith thou threatenest us, if thou art of the truthful.

(40) Then let them produce speech the like thereof, if they are truthful.