The word(s) "truthful" appears 40 time(s) in 40 verse(s) in Quran in Mohsin Khan translation.

(1) And if you (Arab pagans, Jews, and Christians) are in doubt concerning that which We have sent down (i.e. the Qur'an) to Our slave (Muhammad صلى الله عليه وسلم), then produce a Surah (chapter) of the like thereof and call your witnesses (supporters and helpers) besides Allah, if you are truthful.

(2) And He taught Adam all the names (of everything) , then He showed them to the angels and said, "Tell Me the names of these if you are truthful."

(3) Say to (them): "If the home of the Hereafter with Allah is indeed for you specially and not for others, of mankind, then long for death if you are truthful."

(4) And they say, "None shall enter Paradise unless he be a Jew or a Christian." These are their own desires. Say (O Muhammad صلى الله عليه وسلم), "Produce your proof if you are truthful."

(5) All food was lawful to the Children of Israel, except what Israel made unlawful for himself before the Taurat (Torah) was revealed. Say (O Muhammad صلى الله عليه وسلم): "Bring here the Taurat (Torah) and recite it, if you are truthful."

(6) Those (Jews) who said: "Verily, Allah has taken our promise not to believe in any Messenger unless he brings to us an offering which the fire (from heaven) shall devour." Say: "Verily, there came to you Messengers before me, with clear signs and even with what you speak of; why then did you kill them, if you are truthful?"

(7) O you who believe! When death approaches any of you, and you make a bequest, (then take) the testimony of two just men of your own folk or two others from outside, while you are travelling through the land and death befalls on you. Detain them both after As-Salat (the prayer), (then) if you are in doubt (about their truthfulness), let them both swear by Allah (saying): "We wish not for any worldly gain in this, even though he (the beneficiary) be our near relative. We shall not hide Testimony of Allah, for then indeed we should be of the sinful."

(8) Allah will say: "This is a Day on which the truthful will profit from their truth: theirs are Gardens under which rivers flow (in Paradise) - they shall abide therein forever. Allah is pleased with them and they with Him. That is the great success (Paradise).

(9) Say (O Muhammad صلى الله عليه وسلم): "Tell me if Allah's Torment comes upon you, or the Hour comes upon you, would you then call upon any one other than Allah? (Reply) if you are truthful!"

(10) Eight pairs: of the sheep two (male and female), and of the goats two (male and female). Say: "Has He forbidden the two males or the two females, or (the young) which the wombs of the two females enclose? Inform me with knowledge if you are truthful."