The Meaning of Truthfulness
Sidq linguistically: is the antonym of lying. Saddaqah (he believed him): means that he accepted his statement. Sadaqahu al-hadith (he was truthful to him in speech): he told him the truth.  Lisan al-`Arab, Ibn Manzur (10/193).
Sidq technically: is informing about a thing as it is - the opposite of lying.  al-Wadih fi Ususl al-Fiqh, Ibn `Aqil (1/129). The Difference between Truthfulness and Fidelity  al-Furuq al-Lughawiyyah, al-`Askari (p. 585).
It is said that the difference between them is generality and specificity: every fidelity is a form of truthfulness, but not every truthfulness is fidelity. Fidelity may be in deed, not just in statement. Truthfulness may only be in speech since it is a form of announcement, which is always in word.Commandments of Truthfulness and Encouragement towards it in the Qur’an and Sunnah
❖ Allah, exalted, says: “Believers, be mindful of Allah and be with the truthful.” al-Tawbah: 119.
❖ He, sanctified, also says: “Allah will say, ‘This is a Day when the truthful will benefit from their truthfulness. They will have Gardens graced with flowing streams, there to remain forever. Allah is pleased with them and they with Him: that is the supreme triumph.’” al-Ma’idah: 119.
❖ Ibn Mas`ud, Allah be pleased with him said: “The Messenger of Allah ﷺ said: ‘Truthfulness guides to righteousness, and righteousness guides to the Garden. A man continues to say the truth until he is truthful. Lying guides to wickedness, and wickedness leads to the Fire. A man continues to lie until he is written with Allah as wicked.’”  Reported by al-Bukhari (6094) and Muslim (2607). Quotes of the Predecessors and Scholars on Truthfulness
❖ Abu Bakr, Allah be pleased with him, said when he was given allegiance to be caliph: “...Truthfulness is a trust, and lying is treachery.”  Reported by al-Fasawi in al-Mashyakhah (9) and al-Tabari in al-Tarikh (3/210) extensively.
❖ al-Fudayl ibn `Iyad said: “there is no morsel of flesh more beloved to Allah than a truthful tongue, and no morsel more hateful to him than a lying one.”  Rawdat al-`Uqala’, Ibn Hibban (p. 52).
❖ It is also said: “From the prestige of truthfulness is that its possessor is truthful with his enemy.”  `Uyun al-Akhbar, Ibn Qutaybah (2/26). Benefits of Truthfulness and its Impact
Sound creed. Expansion and sacrifice to give victory to the religion. Love of the righteous and the company of the truthful. Steadfastness upon uprightness. Being far from areas of doubt. Being true to one’s promises. Means to Acquiring Endearment
Being watchful of Allah, exalted. Bashfulness. Accompanying the truthful. Spreading truthfulness in one’s family. Supplicating to Allah. Knowing Allah’s punishment for the liars and His torment for the fabricators. Examples of Truthfulness
❖ Our Prophet ﷺ was the most truthful and righteous of people. He was the most complete in knowledge among them, as well as the most complete in faith and certitude. He was known for truthfulness among his people, and none of them doubted that, so much so that he was labelled as ‘Muhammad the Trustworthy.’ Due to this: “When Heraclius, King of the Byzantines, asked Abu Sufyan about those matters about which he asked him of characteristics of the Prophet ﷺ, he had said: ‘Did you use to accuse him of lying before he said what he said?’ Abu Sufyan replied: ‘No.’ Heraclius said: ‘He would not leave lying to the people then start lying about Allah, honoured and majestic.’”  Reported by al-Bukhari (7) and Muslim (1773). See also: Tafsir Ibn Kathir (4/604).
❖ Ka`b ibn Malik, Allah be pleased with him, was saved through truthfulness. In his long narration about his falling back on the day of Tabuk, he said: “... When I heard that he the Prophet ﷺ was on his way back to Madinah, I got dipped in my concern, and began to think of false excuses, saying to myself: ‘How can I avoid his anger tomorrow?’ I took the advice of wise members of my family in this matter. When it was said that Allah’s Messenger ﷺ has approached, I abandoned all the evil false excuses from my mind and I knew well that I could never come out of this problem by forging a false statement. I firmly decided to speak the truth. … Then I came to him, and when I greeted him, he smiled a smile of an angry person and then said: ‘Come.’ So I came till I sat before him. He said to me: ‘What stopped you from joining us? Had you not purchased an animal to carry you?’ I answered: ‘Yes, Messenger of Allah. By Allah, if I were sitting before any person from among the people other than you, I would have avoided his anger with an excuse. By Allah, I have been bestowed with eloquent speech. But, by Allah, I knew that if today I tell you a lie to seek your favor, Allah would surely make you angry with me in the near future, but if I tell you the truth, though you will get angry because of it, I hope for Allah’s pardon. Really, by Allah, there was no excuse for me. By Allah, I had never been stronger or wealthier than I was when I remained behind.’ Then Allah’s Messenger ﷺ said: ‘As for him, he has told the truth. Get up until Allah decides your case.’... Ka`b said: ‘Allah’s Messenger, Allah has saved me due to my telling the truth; so it is a part of my repentance to speak naught but the truth as long as I live.’ By Allah, I do not know anyone of the Muslims whom Allah has helped in telling the truth more than me. Since I mentioned that truth to Allah’s Messenger ﷺ until today, I have never intentionally told a lie. I hope that Allah preserves this for me as long as I remain.”  Reported by al-Bukhari (4418) and Muslim (2769). Forms of Truthfulness
Truthfulness: Of the tongue. Of intention and will. Of resolve. Of fulfilling one’s resolve. In work. In religious matters. Matters that Compromise Truthfulness
Hidden lying: ostentation. It is a hidden form of polytheism. Innovation. Excessive speech. Compromising on one’s own principles: letting oneself do as it pleases. Incongruence between speech and action. Proverbs and Poetry about Truthfulness
❖ “Truthfulness is honour, and lying is abasement.”  Majma` al-Amthal, al-Maydani (p. 408).
❖ The poet said:
“When matters are to be paired up,
Then honesty gives the noblest results.
Truthfulness intertwines on its ally’s
Head a crown, due to his truthfulness.
Truthfulness: its limbs extend out
in all directions as guiding lamps.”  Rawdat al-`Uqala’, Ibn Hibban (p. 53).