It is to conform (to the truth). (Lumaʿ, 288)
Abū ʿAlī al-Daqqāq stated: The pillar of the matter is ṣidq (truthfulness). Every action is completed by it and finds its order therein. Ṣidq occupies a rank immediately below the degree of prophethood.
The lowest degree of ṣidq is the congruence of the inner and the outer. A ṣādiq is a person whose words are truthful. A ṣiddīq is one who is truthful in all words, deeds, and spiritual states (ḥāl).
Ahmad Ḥadraveyh says: Whoever desires that Allah be with him should cling to the principle of ṣidq with all his might. For Allah has said: "Indeed, Allah is with the ṣādiqūn (the truthful)" (al-Baqara 2:153).
Ṣidq is to speak the truth even in a place where one would perish. It is said that ṣidq is for the essence to be in accordance with the word.
Kannād says: Ṣidq has been defined as preventing the unlawful (ḥarām) morsel from passing down the throat.
ʿAbd al-Wāḥid Zayd says: Ṣidq is to show loyalty to Allah by acting (ʿamal) for His sake. (Qushayrī, 105)
Ṣidq is to appear to people as you truly are, or to be as you appear to them. When al-Ḥārith al-Muḥāsibī was asked about the sign of ṣidq, he said: The ṣādiq is the one who, if all his reputation among people were to depart from their hearts—because his heart has found complete righteousness (ṣalāḥ)—would not care at all; if he had good deeds the weight of a mithqāl, he would not desire that people know of them; nor would he be displeased if his bad deeds became known to people.
In both al-Qushayrī's Risāla and its translation (Uludağ, Istanbul 1991, 356), the translation of al-Baqara 2:153 is given as: "Indeed, Allah is with the patient (ṣābirūn)." The verse relevant to this topic is: "O you who have believed, fear Allah and be with the ṣādiqūn (the truthful)!" (editor's note)
For to be displeased at this is proof of desiring increased reputation among people. This is not from the character of the ṣiddīqūn (the utterly truthful). (Qushayrī, 106)
The word ṣidq is used in six senses: ṣidq in speech, ṣidq in intention and will, ṣidq in resolve, ṣidq in loyalty together with resolve, ṣidq in action, and the realization of all the degrees of the religion.