Truthfulness is very closely related to trustworthiness and can perhaps be regarded as its twin quality. Like trustworthiness, it was also the second distinguishing feature of our Prophet, sallallahu 'alayhi wa sallam, as he was called as-sadiq (the truthful) for his immaculate truthfulness. Similar to trustworthiness, it is intertwined with Islamic faith ('iman). It is inconceivable that a believer (Muslim) will not be truthful, or that a liar can be a believer.
In terms of graveness and abomination, the Holy Qur'an equates telling lies to worshiping idols. In Surah Hajj, it says:
"Keep away from the filth of the idols and keep away from telling lies, dedicating yourselves purely and uprightly to Allah, without equating anyone with him." [Al-Qur'an 22:30-31]One day Allah's Messenger, sallallahu 'alayhi wa sallam, stood up immediately after praying Salatu'l-Fajr (the Morning prayer) and repeated three times, "False witness has been equated to idolatry". The he recited the verse of the Qur'an quoted above.{qluetip title=[1]}Reported from Khuraym ibn Fatik in Sunan Abi Dawud and Sunan Ibn Majah).{/qluetip}
Also, consider this - The Messenger of Allah was asked if a believer could be a coward or a miser. He indicated that it was possible. But when he was asked if a believer could be a liar. He replied, "No!" {qluetip title=[2]}Reported from Safwan ibn Sulaym by Malik and Bayhaqi.{/qluetip}
Let us just remind ourselves of the following two ahadith while talking about trustworthiness:
"There are four traits which whoever possesses is a pure hypocrite; and whoever has any one of them, has a trait of hypocrisy until he gets rid of it: When entrusted, he embezzles (cheats); when speaks, he lies; when promises, he reneges (breaks them); and, when quarrels, he abuses (uses foul language)." {qluetip title=[3]}'Abdullah ibn 'Umar in Bukhari and Muslim.{/qluetip}
"A believer (Muslim) might have, being subject to the frailty of human nature, other bad traits but not dishonesty or lying." {qluetip title=[4]}Reported by Ahmad and Bayhiqi from Abu Umamah.{/qluetip}
Hence, the Messenger of Allah, sallallahu 'alayhi wa sallam, admonished:
"You must adopt truthfulness because it leads to piety which leads to Jannah (paradise). When a person keeps telling the truth and adopts a life of truthfulness, he is written as a siddiq (the most truthful and dedicated to truth) in the records of Allah . And you must stay away from lying because it leads to wickedness which leads to the Fire. When a person keeps lying and indulges in lies, he is included in the list of liars with Allah." {qluetip title=[5]}Reported from 'Abdullah ibn Mas'ud in Bukhari and Muslim.{/qluetip}
"The gravest dishonesty is that you lie to your (Muslim) brother about something while he believes that you are telling the truth." {qluetip title=[6]}Reported from Sufyan ibn Asid al-Hadhramiy in Abu Dawud.{/qluetip}
On the positive side, let us also read three ahadith that give good news to the truthful people:
"If loving Allah and His Messenger or being loved by Allah and His Messenger pleases a person, then he must tell the truth whenever he speaks, give back the trust when entrusted, and behave superbly to his neighbours." {qluetip title=[7]}Reported by Bayhiqi in Shu'abu'l-Iman from 'Abdur-Rahman ibn Abi Qurad.{/qluetip}
"You guarantee me consistent practice of six actions, I will guarantee you Jannah (Paradise): Be truthful when you speak, fulfill promises when you commit, pay up when you are entrusted, protect your private parts, lower your gaze (turn away from inappropriate sights), withhold your hands (from doing improper things)." {qluetip title=[8]}Reported by Bayhiqi in Shu'abu'l-Iman from 'Ubadah ibn Samit.{/qluetip}
"A truthful and honest businessman will be in the company of prophets, their sincerest companions and martyrs." {qluetip title=[9]}Abu Sa'id in At-Tirmidhi, Ad-Darami, and Ad-Daruqutni.{/qluetip}
Not only what we say must be the truth, but also it must be what is the most appropriate, fair, reasonable, to the point and beneficial. The Holy Qur'an commands:
"O believers! Adopt taqwa and say only the words that are in keeping with the faith (befitting, right to the point)." [Al-Qur'an 33:70]
A sincere believer, in addition to being extremely careful in avoiding falsehood in serious situations or major things, also watches what he says. He avoids saying anything that is not true even in minor situations, in speaking to little children, or in jest. He even avoids passing on rumours, hearsay or anything for which he himself does not have personal knowledge of its accuracy.
The Messenger of Allah, sallallahu 'alayhi wa sallam, said:
"It is enough of a lie for a person to tell others what he hears." {qluetip title=[10]}Reported in Muslim from Abu Hurayrah.{/qluetip}
'Abdullah ibn 'Amir stated that one day the Messenger of Allah was sitting in our home and my mother called me, "Come here, I will give you something." The Messenger asked, "What do you want to give him?" She said that she wanted to give him a date. He responded, "If you do not give him anything, it will be recorded against you as a lie." {qluetip title=[11]}Abu Dawud{/qluetip}
"Lying is not acceptable, whether seriously or jokingly. Not even in case when you promise to give your child something and then do not give." {qluetip title=[12]}Reported in Al-Adab al-Mufrad from 'Abdullah.{/qluetip}
"Woe to him who tells lies to make people laugh. Woe to him. Woe to him." {qluetip title=[13]}Baz ibn Hakim in At-Tirmidhi.{/qluetip}
Giving a false impression is also a form of lie:
A woman asked the Messenger, "My husband has another wife. Will it be sinful if I dress and live well, making it appear as if all of that is being provided by my husband?" He replied, "If one gives that impression for the things he does not give, then that person is like someone who dresses up in lies." {qluetip title=[14]}Reported in Bukhari and Muslim and Mishkah from Asma.{/qluetip}
Often, when offered something, people pretend as if they do not need it, while in reality they do. That also is regarded as a lie:
When we brought one of the Prophet's wives to him as a bride, he brought out a bowl of milk, drank some and then gave it to his wife. She declined saying, "I have no appetite." He responded, "Do not add lie to your hunger." {qluetip title=[15]}Reported by Tabarani from Asma bint 'Umays.{/qluetip}
However, there are some exceptions where withholding the truth, obfuscating and, in some situations, even misrepresentation does not make the person a liar. One such situation is during war. For example, if a Muslim is caught as a prisoner of war, he must not tell the true position and facts about the Muslim army that will be beneficial to the enemy. Rather, he should say things that will misguide the enemy or discourage it in order to shorten the war, to reduce the fatalities in the war or to help Muslims win. Another such situation is when reconciling two parties. The Messenger of Allah said:
"He is not a liar who says some good things and conveys good comments to reconcile among the people." {qluetip title=[16]}Reported from Umm Kulthum in Bukhari and Muslim.{/qluetip}