As for truthfulness (sidq), Allah says:
“O you who believe! Have fear of Allah (taqwa) and be with the truthful.” (9:120)
“Be among the truthful” because the one who sits with someone is influenced for good and evil. Nature is a thief. A man copies the one he mixes with a lot, even if he is not aware of it. Tarafa said,
Do not ask about a man. Ask about his close companions.
Every man imitates his close companions.
This is the best path to travel for someone who wants to discipline himself. It is to mix with the perfect man so that his nature and good habits flow to him. The influence of actions is absolutely stronger than the influence of words.
Al-Qushayri noted, “Truthfulness is the basis of the matter and it is made perfect by it, and it is commanded.” He said, “The least truthfulness is when what is secret and what is public are the same.”
Sahl said, “The scent of truthfulness has not been smelt by any person who flatters wither himself or another person.”
“Truthfulness is the sword of Allah,” said Dhu’n-Nun al-Misri. “It is not applied to anything but that it cuts right through it.”
Al-Harith al-Muhasibi said, “The truthful man is the one who does not care if his entire worth in the hearts of men should vanish away if it is for the sake of putting his heart in harmony.
He does not like people to know about his least good actions, and he does not dislike people being aware of his bad actions. If he dislike that, it is proof that he desires to be greater in their eyes. This is not sincerity of the truthful ones.”
It is said that if you seek truthfulness from Allah, He will give you a mirror in which you can see the baseless falsehood of this world and the Next.
The master, Abu’l-Qasim al-Junayd, said, “The truthful man is turned about forty times a day, while the show-off is fixed in the same state for forty years.”
By this, he means that the truthful man changes direction with the truth, no matter what it is. When sees something in the Shari’a which is considered to be excellent, he promptly does it, even if it is at variance with his customary habit. If something arises in the Shari’a which is more important, but he cannot combine the two things, then he moves to the better one.
He continues to behave like that. In a single day, he may have a hundred different states, or a thousand or even more, according to his mastery of gnosis, and the manifestation to him of fine points and subtleties.
The show-off, on the other hand, clings to one state alone, and if something arises which the Shari’a considers more important in some states, then he will still not do that important thing. He will hold fast to his state, because he is showing off his worship and his state to creation.
He fears that people will stop loving him if he changes. He is very careful to ensure that their love continues. But the truthful one desires only the Face of Allah by his worship. Whenever the Shari’a gives greater weight to a certain state, he goes to that state, and does not turn to creation.
-Imam Abu Zakariyya Muhyi’d-din ibn Sharaf an-Nawawi, The Garden of the Gnostics (Bustan-ul-Arifeen